So a 4" flared would be fine as well? Assuming I wire to 4 ohm
Not sure what you're asking here. One 4" flared per driver wouldn't hurt, if you don't mind the extra vent length and displacement. I'd probably opt for 3" flared instead, particularly to the extent that I though I wouldn't need to push them hard.
At 36.2 m/s, you could get away with one 4" flared for both (one 1.5 ft^3 airspace, 340 watts combined). However, it is generally preferable to have separate airspace for each vented driver.
Not sure how wiring configuration/load impedance figures into vent selection.