to wire both of them together, you need to upgrade the size of the wire you use, as it will draw more juice. its perfectly reasonable to do that.
the reason you dont, is as juinor said. if you have a relay go bad, or some other reason that juice stops flowing through those wires, your truck is dead alongside the road. if you wire them seperate, you might have low fuel pressure, but can baby it home.
imo, two stockers is better than the fuelab, for that exact reason. it doesnt strand you right where you are, like tarms almost did to him.