I suggest memory foam as well. But they get warm. If that doesn't work, consider the following?
Buy two smaller beds and slide them side by side. I think a King bed is two double beds side by side. 2x twins is a queen. No needs for modifications of anything, just slide both mattresses side by side into a king sized bed frame.
Cheaper, easier to transport (somewhat) and you can choose varied stiffness or even separately choose memory foam and coil if you want. Only thing you may need to do is make sure the height matches. One side really shouldn't affect the other side. If it does, maybe slot some foam in between the bed frame and the mattress so it cannot shift as much?
http://www.amazon.com/Create-a-King-.../dp/B0007KNKN2 See this link for a pic. I don't think you need this thing in the middle though.