We have installed (haven't finished the plumbing
2000 pumps one over the other, but they will have separate discharge lines. The upper pump will only come on if the lower pump is overwhelmed (we have a major problem).
If these pumps went to a common manifold and then out a single line what would keep the water from getting to the manifold and then trying to go downhill (the route
of less resistance) back to the other pump. Only a check valve would prevent that and one isn't suppose to have one of those in the line.
Another hole isn't going to be a big deal as it is going to be above the waterline. At least ours are. Our boat
came with one manual and one electric
and two discharge hoses. I'll drill the hole and add the third discharge hose.
Here is our install to this point and I'll finish the new discharge lines when we return. Also there will be an indicator light on when the bottom pump is running and an audible alarm
if the top pump ever kicks on......
Endeavour Plumbing page 16