I'm pretty sure an imminent attack on an inquisition stronghold or some other dire threat to the race of humanity (of which many abound) would be sufficient reason for the 2 to work together. A threat that will annihilate 2 squabbling parties generally is enough for those parties to work together against it to ensure their survival. This seems to be the general blanket story behind many apoc games anyhow.You can do that in Apoc, but its not really in the "spirit" of the game. Army mixes in Apoc should make sense fluff wise, so if you can come up with a reason than its okie, but shouldn't be just put together for the sake of it being totally awesome. If you did use a radical inquisitor with demonhosts I would add something about the inquisitor not being to benefit from any greyknight abilities and vice versa or that if a greyknight unit is within a certain distance of the demonhost there is a chance that they will attack that unit instead.
Speaking of which, when the make a new Daemonhunters codex they should let radical inqusitors use some items from the chaos codex.mainly the ones that use demon weapons and demonhosts.