I tried quarter-wave plate and saw no loss. BY tilting the horns you can get the goal but you must test the full radiation pattern to check coupling.
Using dual-polarized horns is possible but again you must know the full radiation pattern (not only E and H) to check coupling.
Dual-polarized horn needs to control amplitude and phase to specify the tilt, and all affects the radiation pattern.
I would prefer to experiment, take two horns and see what happens and what works best.