Can you cut the THG heat tape in the middle? Which heat tape are you using?
The Flexwatt, which is actually 11" not 12" cannot be cut into 7.5" panels. It has to be cut in certain areas, which are about 12" wide.
It can be cut anywhere in between the strips that run across it, it doesn't have to be 12 inches long.
I have cut flexwatt into shorter strips many times over the years and as long as the strips on each shelf are the same size they work just fine.