skater (non-goalie) bag + stick can fit easily w/the top off. I've thought about turning the bag upright and strapping it in w/the seatbelt, but haven't tried it yet. Seems like it should work too.
You could also split the gear as you suggested. No loose skates in the boot, though - too much weight w/sharps flying around :no:...Once you're done playing for the day and re-pack gear in the boot, it'll be sweaty, and over time, your boot might start smelling like your bag. I suppose you could keep some towels back there.
So, yes, it can be done and quite easily so don't worry about it.