the cheapest way is to switch cassettes on the trainer, but that’s far from convenient…
I would try to force ERG mode on 1 of the bikes (with a different cassette). The bike with the matching cassette can be used in all gears.
You can use TR just fine in ERG only.
Upgrading from 10 to 11 speed will cost you quite some… (chain, cassette, derailleur, shifter). But that’s the most complete solution