When it comes to radio electronics nothing is impossible as long as you have the funds and the know how. It is really basically manipulating electrons and passing audio from one device to another device. Crossbanding in-band and out-of-band multiple radio devices can be accomplished in several ways, audio tie with a logic controller, voting comparator using COR, console patch, or an interoperability gateway.
I've designed several crossband package configurations back in the 70s, 80s, and early 90s and some of them have been written up in various trade magazines. Back in the early 80s, I designed an airborne repeater package using a GE Porta-Mobile II Low-Band Radio and a GE MPD Portable with a Sweet Electronics Logic Controller and put it in an Airplane so two vehicles (car on a VHF Highband Channel and a motorcycle on a Lowband VHF) can directly communicate over 15-20 miles apart in hilly terrain while in motion traveling across the State for a special event. "Some People" not in the know couldn't figure how in the hell I was able to make a Lowband radio talk directly to a Highband radio. My answer to them was "I used the "PFM" technology method.
I truly missed those days creating solutions like this to make the impossible possible.