One of the engines read 10 - 12 psi and the other reads 6-8 at cruising speeds? Any idea what could be causing the difference. Drives are lower units are new, impellers new, sea strainers clean, guages replaced, Stelling headers pressure checked fine.

Is there a way to control water pressure at the water pump or by controlling the flow of the water dump at the headers?

are the belts on the water pumps nice and tight? getting any slippage? How about the engine blocks? free of debris?
You can get a water psi relief valve from merc if it getting to high. But if everything is working properly and you are getting enough water coming out of the pipes when running and motors are not over heating i wouldnt worrky about it. You will probably never get exact same readings only because nothing is built exactly the same. Maybe the gauges are not reading right....

Switch the gauges first to make sure thats not the problem. You could also have a partial obstruction near where the pressure is taken from.