I have no experience with sand, so I do not know about Siphoning it.
If you have a Filter on 1 side, and it's low flow, I think that would be okay.
Otherwise, you could just baffle it, even if it's Low Flow.
For the Heater, you could just put it near the Divider so the other fish could get more warmth. If you are concerned about who gets more heat, just get a Heater that you can adjust. Just change it to 80F-82F. It's better more Hot than more Cold. Splitting the tank would be perfectly fine. Just make sure that the Divider isn't see through, and that they can't jump over it. An extra 2 inches would be okay. Good Luck with the new idea.
If you have a Filter on 1 side, and it's low flow, I think that would be okay.
Otherwise, you could just baffle it, even if it's Low Flow.
For the Heater, you could just put it near the Divider so the other fish could get more warmth. If you are concerned about who gets more heat, just get a Heater that you can adjust. Just change it to 80F-82F. It's better more Hot than more Cold. Splitting the tank would be perfectly fine. Just make sure that the Divider isn't see through, and that they can't jump over it. An extra 2 inches would be okay. Good Luck with the new idea.