Ours is two separate compressor / coil units with a shared circulating pump.Also do you have 2 independent systems, or a split system? (A split system has 1 compressor + an evaporator in each cabin.)
On ours the rear unit does have less pressure on the discharge. First it is farther away from the pump and distance creates head which will cause the water to seek the course of least resistance - out the forward discharge. I do my own air conditioner winterizing and I can tell you that the through hull for the aft unit is smaller than the forward unit - more head pressure forcing more water out the path of least resistance. Next your comments on blockage are dead on although your don't ask comment has me visioning either a closed valve or water spraying everywhere ? so we might have to ask. But just in case check the through hull valve for the aft unit discharge to make sure it isn't partially closed. It is located in the starboard stern locker and if you've got stuff stored back there it is entirely possible the valve got partially closedI would think you're talking about the water coming out the thru hull that takes away the heat from the condenser in each unit. On my 41 there is a single pump located in the bilge section in front of the engine that Tees and feeds both units. There can be any manner of blockage in the line leading to the aft unit. You can remove it from the Tee and put a garden hose down it to increase the pressure and clean it out. I usually limit the amount of water pressure so as not to cause damage. Although I have gone at it full blast on one occasion when I was truly annoyed with it with out incident. There didn't happen to be a blockage at that time. Don't ask.