Common sump between the two tanks (ie. connected)?
If not then you can't really use a single calcium reactor to feed both.
You can use a kalkreactor to feed multiple tanks by T'ing off the output as many times as you need tanks, since they work on the principle of adding (or replacing) water that is lost to evaporation. So you can add your effluent to as many tanks as you want so long as you can fine tune the rate of addition for each tank.
But a calcium reactor works by drawing in tank water and then replacing it. It recirculates the water through the media and then lowers the pH enough to dissolve the media, and replaces the water in.
So you'd have to figure out a way to draw water in from both sources, and replace the exact same volume to each. Essentially it's not really possible. It would be like trying to run a cannister filter on two disparate tanks.
It's hard enough trying to run two calcium reactors off one CO2 cylinder. Now that would be an interesting challenge. Difficult, but shouldn't be impossible. The challenge would be figuring out how to deliver the right amount of gas past the Y splitter. If you just had two needle valves you could play with the amounts, but recognizing that if you open one up, there's less gas going the other way, and vice versa, closing one up means there's more gas going the other way. But I bet it could be done if one were patient enough. Alternatively, one could put the Y adaptor on the tank itself, and have a regulator and needle valve for each output. That way monkeying with one of the needle valves would not affect the flowrate of the other (assuming a high enough upstream pressure, things might go to pot once the cylinder starts to empty). Anyhow these are just my ideas and there may be pitfalls with trying them that I don't know about. But just some food for thought. If you could figure this out though it'd be awesome because then you could just have one large cylinder servicing multiple needs (heck even splitting CO2 between SW and FW tanks for example).
Unfortunately when it comes down to multiple tanks needing a calcium reactor, I think it's far easier just to have two calcium reactors (I actually have two reactors and two cylinders myself for this very reason, although at the moment I'm only running one on one tank, but at one point I did have two going).
PS. Now if you had the two tanks on the same sump though, then basically you just have the one system, and can use the reactor to feed both display tanks since they're interconnected anyhow. There's one argument for running different tanks on the same sump, an economization of hardware.
