yes separated is ideal. but when you fill you will get uneven pressure because of the location of the valves and length of the tubs to the bag.
additionally you have the same problem with the deflate.
(the above is in ref. to individual fill and individual dump valves. )
Therefore: one fill and one dump is the easiest to install.
I just wish there was a highly restricted coupler/fitting that would highly slow the transfer of air from side to side, that would give some time to make the cure and level the truck side to side, before much sway.
I have about 1800lbs in the bed (all the time), so when i raise it up to about factory
I believe that we lose the anti sway part built/designed into the leafs to help prevent the sway.
Air pressure is the same regardless of length of line or diameter. The time to fill will change with restriction.