How would a single thermostat decide to fire up the hot air furnace rather than the boiler?
At equivalent AFUE numbers, heating with the boiler is inherently more efficient than ducted hot air, due to FAR lower distribution losses. If the hot air furnace is located in an attic above the insulation layer, the difference is even greater, since none of the standby and distribution losses accrue to the house, only to the attic. It's enough of a difference that even an 80% AFUE boiler in a semi-conditioned basement would cost measurably less to run than a condensing furnace in the attic.
Seems like a lot of trouble to go through for just a backup system, but just leave it as a separate system, and leave the AC/hot-air thermostat in cooling mode unless you ever actually need it. There's zero advantage to making a shot-gun marriage between the hydronic & hot air systems. I suppose you could do it with a multi-stage thermostat, making the hot air furnace the second stage, but it's just never going to be worth it.