Heat gradient?
Another consideration is how you plan to create a heat gradient. The easier way (I think) to creeate a heat gradient would be to divide the viv into two narrow, long areas and have the heat source (UTH I assume) on one side. If you divide it into two square-ish portions, then you may want to put the heat in the middle and put the divider on top of it splitting the heat source that way you still only need one heat source.
I have read, and it makes sense, that you dont neccessarily NEED a heat gradient, but who wants to be stuck at one temperature without control to change it.
Either way, let us know how it works out and which way you decide to go.
Jimmy