Possibly the fastest way to make 5 gallons of Imperial Stout and 10 gallons of Irish Stout using a single 15 gallon vessel without extract.
BIAB all grains except for 6oz roasted barley and 4oz pale chocolate malt in 9 gallons of mash water. Mash for 25 minutes, stirring frequently. Lift and squeeze bag. Drain kettle/mash tun into buckets or something. Put another 9 gallons of (preferably preheated) water into the kettle, heat to mash temp, reinsert the full grain bag, add the remaining grains and sparge/remash for 15 minutes. Lift bag and squeeze.
There should now be one wort with sg over 1.074, and another wort with sg less than 1.039. It should in theory be possible to add some of the heavy wort to the light wort, and some of the light wort to the heavy wort, to create something in the neighborhood of 6-6.5 gallons of 1.074 wort, and 11-11.5 gallons of 1.039 wort.
Boil the 1.039 wort for 30 minutes with one ounce of Magnum. Add two pounds of maltodextrine at some point during the boil. Chill and move to fermenters. Post-boil sg target 1.045.
Boil the 1.074 wort for 30 minutes with 2 ounces of Magnum. Add one ounce of Huell Melon with a few minutes remaining in the boil. Chill and move to fermentor. Post-boil sg target 1.089.