Based on the Amel Appendix sketch of seawater flow, if the aft cabin AC unit heat exchanger was plugged, then the other two units would not get any flow. Also, if the AC HX was blocked, would not the seawater pump would trip its breaker? Also, if the two forward AC's were not getting any flow they would trip their breakers (you can check this by closing the seacock, which I did once inadvertently).
This doesn't seem like a good idea on the face of it to me (but then what I don't know would fill volumes...). However it seems to work pretty well.