Can both pumps be run individually? Do they both have a common inlet port, in other words do the both feed off the same inlet. Try running them seperate first and see if they both act the same individually as they do when you run them together. This can help diagnose if you have a blockage, because a single pump running may be able to get enough volume through a blockage to operate properly where two running may resist flow because of restricted volume, also try this...take a barrel bore brush from a rifle cleaning kit and put it on the ram rod that comes with the kit. Run that down the inlet line from outside the boat and see if you get anything. Sounds like you may have a blockage of some sort. Almost anything that is caught in that inlet will stick to the barrel brush and be pulled out when you remove it. If you don't get anything then I'd say its time for pumps.......
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Isaiah 40:31 "but those who hope in the Lord will renew their strength. They will soar on wings like eagles, they will run and not grow weary, they will walk and not be faint.