This could be MAJOR problem... As stated above water will take the path of least resistance.
I have three factory installed Marine
Airs units on my boat... Two of them are on one water pump and the third has its own pump. All of them are run on 1/2" water lines and none of the untis have a "back-up water line".
Several years ago the water line plugged in the two unit water intake and I fried one of the AC units from lack of water flow. I had the same symptom as you... one flowed normal the other flowed at a trickle.
It turned out a critter got behind the intake grate and grew large enough to obstruct the intake of salt water
to about 50%. The unit with the least head
pressure got the majority of the water flow.
Check the the salt
water system from the intake, including the raw water
filter, the plumbing
and the outbound through hole.
My guess is you have an obstruction somewhere....