Right, if you want to stack RF decks and have a single control head running both of them, you need the TK-x90 series (analog only), the TK-5x10 series (analog/P25), or wait a few months and get the NX-5000 series mobiles (analog/P25/NXDN)
The NX-5000 series mobiles will allow you to stack up to 3 RF decks and have one or two control heads. The other models will do combinations of one or two control heads with one or two RF decks.
I've got an NX-700 (VHF) and an NX-900 (800MHz) in my work truck. While a single head would be nice, I actually prefer having two separate control heads. Let's me listen on both systems at the same time, and not miss anything.