I'm the wife of a serious Air Hog. He used to take my air the first 15 years of diving, and I did not like it very much, you can't move free with someone hanging on your octopus, and watching the own air go down super fast is no fun. Of course he took 15l tanks if available (still not enough on deep dives) - I had to take 15l tanks myself to provide more air for both of us.
Since 10 years he uses two tanks - like 12l and 7 l. or two 11 l. He uses a normal BCD and attaches the second tank on the front side. He has a second regulator with it. In the first tank, he leaves the air for emergency, safety stop etc. Works out well.
I always make clear, if its OK for the dive shop. If not, we don't dive there.
This sounds complicated, but is actually much more relaxed than the "air sharing, end the dive early without the group, stay above erybody etc. thing".
Of course It might not at all be necessary for the OP, since he did not yet try a 15 l.