the 50 is tempered, so I cannot drill it, so I'm not sure how I would connect a 40 sump to it. I hate running just one pump in case one goes out, too, but I guess the FX5 would still be running, at least theoretically.
I wonder how much bio-load the current sump and FX5 can handle (I'm at 6x turnover right now). There aren't any other holes where I can run another FX5 or a 2260/2 effectively, either, but I could probably rig something up in the overflow box.