I used to have two seperate 5g tanks squeezed into one frame meant for a tank twice the size, but they were both betta tanks. We used mountable false stone backgrounds on each dividing wall (and the backs) and it turned out pretty sweet. I ended up giving one of the tanks to a friend to take to college.
Why not take it to the next step and make 3... Freshwater, brackish, saltwater.
I wouldn't worry much about the temperature, personally. If you monitored it closely and it stayed consistant, you might be able to utilize the heat from your SW
tank and use it to heat your FW
tank rather than trying to suppress it.
(PS, nice color on that bay. Reminds me of my mare.