If you were trying to grow out your corals for fragging I would seperate them but if this is the "sale side", if that makes sense, I would just plumb them together as well. The reason I say this, as you already know, LPS and softies thrive in dirtier water than SPS. So in the long run you could frag more LPS and Softies from less prestine water than your SPS tank. But if you are just selling these items versus growing them out I dont think it would really be worth trying to keep two systems up and running.
Hope that made sense. lol
Kevin- 375 Gallon Reef
Reefing made easy...