Split Mash 11B Bitter & 19A Amber Ale

I have not tried this yet.

Two beers from a single mash using BIAB:
11B Best Bitter | SRM 12 | IBU 34 | ABV 4.3%
19A American Amber Ale | SRM 12 | IBU 40 | ABV 5.9%

Grist needed:

• 13 lbs Briess pale ale malt 3.5L
  
• 1 lb Briess caramel malt 80L
  Steeping grains needed:
  
• 8 oz Briess victory malt 28L (for 11B) & grain bag for steeping
  Water needed:
  
• 9.5 gallons RO with salts/acid per schedule below
  (Reserve 0.75 gallons of treated water for steeping grains)
  Hops & yeast needed:
  
• 1 oz Kent Golding (5.0%)
  
• 1 oz Amarillo (8.1%)
  
• 1 oz Cascade (5.0%)
  
• 1 pack Safale S-04
  
• 1 pack Safale US-05
  
• 2 Whirfloc tablets
  
  Treat water & mash:
  Add salts/acid to 9.5 gallons RO water to create balanced profile:
  
• 4 g Gypsum
  
• 1.9 g Epsom salt
  
• 2.4 g Canning salt
  
• 2.5 g Calcium chloride (dihydrate)
  
• 2.4 g Slaked lime
  
• 4 ml Lactic acid 88%
  Reserve 0.75 gallons treated water for specialty grains
  Mash grist in 8.75 gallons treated water at 152°F for 60 minutes
  Drain bag and collect 8 gallons wort, target SG 1.041
  Split collected wort:
  
• 3.5 gallons for 11B Best Bitter
  
• 4.5 gallons for 19A American Amber Ale
  
  Brew 19A American Amber Ale:
  Bring 4.5 gallons 1.041 wort to boil and follow hops schedule:
  
• 0.5 oz Amarillo (8.1%) @ 60 min
  
• 1 oz Cascade (5.0%) @ 10 min
  
• 1 Whirfloc tablet @ 10 min
  
• 0.5 oz Amarillo (8.1%) @ 5 min
  Target ending kettle volume: ~3.5 gallons
  Chill to 72°F and pitch Safale US-05
  Primary at 64-72°F for 3 weeks
  Fine (optional) and crash 3-4 days then keg
  Carbonate to ~2.6 vols
  
  Brew 11B Best Bitter (start when 19A reaches boil):
  Steep specialty grains:
  
• Bring 0.75 gallons reserved water to 160°F
  
• Put 8 oz Briess Victory malt in grain bag and steep 20 minutes
  
• Drain and squeeze grain bag
  Add steeping water to 3.5 gallons @1.041 collected wort
  Bring to boil and follow hops schedule:
  
• 0.75 oz Kent Golding (5.0%) @ 60 min
  
• 0.25 oz Kent Golding (5.0%) @ 30 min
  
• 1 Whirfloc tablet @ 10 min
  
• 0.5 oz Kent Golding (5%) @ 5 min
  
• Target ending kettle volume: ~3.25 gallons
  Chill to 68°F and pitch Safale S-04
  Primary at 64-68°F for 3 weeks
  Fine (optional) and crash 3-4 days then keg
  Carbonate to ~2.0 vols or less (1-1.5 vols is traditional)
  
  Assumptions:
  
• BIAB method
  
  • absorption 0.05 gallons per pound of grist
    
  • bag is squeezed to drain wort
    
• Need at least 10 gallon kettle or cooler for mashing
  
• Boiloff rate is 1 gallon/hour
  
• Package in kegs
  
  11B Best Bitter from 2021 BJCP:
  
  Overall Impression: A flavorful, yet refreshing, session beer.
  Some examples can be more malt balanced, but this should not
  override the overall bitter impression. Drinkability is a critical
  component of the style.
  
  Aroma: Low to moderate malt aroma, often (but not always)
  with a low to medium-low caramel quality. Bready, biscuit, or
  lightly toasty malt complexity is common. Mild to moderate
  fruitiness. Hop aroma can range from moderate to none,
  typically with a floral, earthy, resiny, or fruity character.
  Generally no diacetyl, although very low levels are allowed.
  
  Appearance: Pale amber to medium copper color. Good to
  brilliant clarity. Low to moderate white to off-white head. May
  have very little head due to low carbonation.
  
  Flavor: Medium to moderately high bitterness. Moderately
  low to moderately high fruity esters. Moderate to low hop
  flavor, typically with an earthy, resiny, fruity, or floral
  character. Low to medium maltiness with a dry finish. The malt
  profile is typically bready, biscuity, or lightly toasty. Low to
  moderate caramel or toffee flavors are optional. Balance is
  often decidedly bitter, although the bitterness should not
  completely overpower the malt flavor, esters and hop flavor.
  Generally no diacetyl, although very low levels are allowed.
  
  Mouthfeel: Medium-light to medium body. Low carbonation,
  although bottled examples can have moderate carbonation.
  
  Comments: More evident malt flavor than in an ordinary
  bitter; this is a stronger, session-strength ale.
  
  History: See comments in category introduction.
  Characteristic Ingredients: Pale ale, amber, or crystal
  malts. Most contain sugar. May use a touch of caramel or dark
  malt for color adjustment. May use corn or wheat. English
  finishing hops are most traditional, but any hops are fair game;
  if American hops are used, a light touch is required.
  Characterful British yeast.
  
  Style Comparison: More alcohol than an ordinary bitter,
  and often using higher-quality ingredients. Less alcohol than a
  strong bitter. More caramel or base malt character and color
  than a British Golden Ale. Emphasis is on the bittering hop
  addition as opposed to the aggressive middle and late hopping
  seen in American ales.
  
  Vital Statistics: OG: 1.040 – 1.048
  IBUs: 25 – 40 FG: 1.008 – 1.012
  SRM: 8 – 16 ABV: 3.8 – 4.6%
  
  Commercial Examples: Adnams Southwold Bitter, Fuller's
  London Pride, Harvey’s Sussex Best Bitter, Salopian Darwin’s
  Origin, Surrey Hills Shere Drop, Timothy Taylor Landlord
  Tags: standard-strength, amber-color, top-fermented, britishisles,
  traditional-style, amber-ale-family, bitter
  
  19A American Amber from 2021 BJCP:
  
  Overall Impression: An amber, hoppy, moderate-strength
  American craft beer with a malty caramel flavor. The balance
  can vary quite a bit, with some versions being fairly malty and
  others being aggressively hoppy. Hoppy and bitter versions
  should not have clashing flavors with the caramel malt profile.
  
  Aroma: Low to moderate hop aroma reflective of American or
  New World hop varieties (citrus, floral, pine, resin, spice,
  tropical fruit, stone fruit, berry, or melon). A citrusy hop
  character is common, but not required. Moderately-low to
  moderately-high maltiness, usually with a moderate caramel
  character, that can either support, balance, or sometimes mask
  the hop presentation. Esters vary from moderate to none.
  
  Appearance: Deep amber to coppery-brown in color,
  sometimes with a reddish hue. Moderately large off-white head
  with good retention. Generally quite clear.
  
  Flavor: Moderate to high hop flavor with similar
  characteristics as the aroma. Malt flavors are moderate to
  strong, and usually show an initial malty sweetness followed by
  a moderate caramel flavor and sometimes toasty or biscuity
  malt flavors in lesser amounts. Dark or roasted malt flavors
  absent. Moderate to moderately-high bitterness. Balance can
  vary from somewhat malty to somewhat bitter. Fruity esters
  can be moderate to none. Caramel sweetness, hop flavor, and
  bitterness can linger somewhat into the medium to full yet dry
  finish.
  
  Mouthfeel: Medium to medium-full body. Medium to high
  carbonation. Overall smooth finish without astringency.
  Stronger versions may have a slight alcohol warmth.
  
  Comments: Can overlap in color with darker American pale
  ales, but with a different malt flavor and balance. A range of
  balance exists in this style, from balanced and malty to more
  aggressively hopped.
  
  History: A modern American craft beer style developed as a
  variation from American Pale Ales. Mendocino Red Tail Ale
  was first made in 1983, and was known regionally as a Red Ale.
  This served as the progenitor of Double Reds (American Strong
  Ale), Red IPAs, and other hoppy, caramelly beers.
  
  Characteristic Ingredients: Neutral pale ale malt. Medium
  to dark crystal malts. American or New World hops, often with
  citrusy flavors, are common but others may also be used.
  Neutral to lightly estery yeast.
  
  Style Comparison: Darker, more caramelly, more body, and
  generally less bitter in the balance than American Pale Ales.
  Less alcohol, bitterness, and hop character than Red IPAs. Less
  strength, malt, and hop character than American Strong Ales.
  Less chocolate and dark caramel than an American Brown Ale.
  
  Vital Statistics: OG: 1.045 – 1.060
  IBUs: 25 – 40 FG: 1.010 – 1.015
  SRM: 10 – 17 ABV: 4.5 – 6.2%
  
  Commercial Examples: Anderson Valley Boont Amber Ale,
  Bell’s Amber Ale, Full Sail Amber, North Coast Red Seal Ale,
  Saint Arnold Amber Ale, Tröegs Hopback Amber Ale