don't try to make enclosures for your door speakers, just seal the doors properly, get a good baffle and call it a day. Car audio speakers are designed for off axis response, so while aiming them will be cool and everything.....it's a lot of work for not a lot of benefit IMO

If you can, find an amp that has a pass through on the signal. So you can run the one preout to the "high" amp and then have it pass through to the low pass amp. That's one option and will take a crossover out of the signal chain....

BTW, all the best amps have no crossovers in them...not the other way around.