How to heat a custom tank

[Slim]

The light is hanging above the glass top outside the cage.

Then what your really heating is the glass, not so much the cage.

To be brutally honest, heating that type and size of enclosure for a Ball Python is going to be tough. An under tank heating element and a light are not going to be optimal, and the lamp is going to kill your humidity if you switch to a screen top, and is not really going to heat the cage if you stick to glass. If you use the suggestion that Aaron gave you about the radiant heat panel, you'll at least stand a fighting chance.

Does this cage have a divider, or are you housing them together? I would advise against housing them in the same enclosure...even a big one like yours.