Am using Multi-Instrument objects in the Environment, and all 16 channels, to boot.
I tried Create Track Stack but that's just a "group", I can't select the group and then play the instruments inside.
Yes this should work. Make sure to create a summing stack. Put two tracks inside the stack with instruments on each. Select the track header for the track stack and play your keyboard...all the instruments will make sound. You can record midi to that tracks tack track, also.
Otherwise, there is always the environment. You don't necessarily need to use a multi-instrument for stacking instruments. Just cable up some channel strips with a single instrument object ahead of it. Multi-instruments are really more about setting up multi-timbral scenarios.
5,1 MacPro 3.46ghz x 12 128gb ram, OSX 10.14, Logic Pro 10.4.8, Mainstage3, Cubase10, StudioOne, Reaper, DP9, VEP, VSL, too many plugins to list