I was looking around at different tanks, both SW and FW, and I had a crazy idea. You may know where I'm going with this. This is a complete idea, and I know that there is a very high chance that it would never work, but hey, why not ask?
A tank that is split in half, with one side being a reef, and the other being FW. Say you get one light that is sufficient for both reef and FW plants. You would silicone like a 3/4"-1" black acrylic all the way to the brace of a 55/75G tank, right in the middle, so no water gets over anywhere. Different filters (one for FW and one or SW), different fish, different enviroments. About what, 27 gallons each?
I'm going to make a picture. I think this would look very interseting IMO. Let me know what you think.
*Remember, idea. No one said I am, or would ever do it (but it is tempting...)*