Ditto neutering the male, if they're killing their joeys they shouldn't be breeding.
When I had some abandoned gliders dump joeys on me, I had to use an oral syringe and let them lick it off of the end. They need to be fed every 2 hours round the clock. I had to use a wet q-tip to help them defecate/urinate. I'd rub the little "nub" and that would usually trigger one or both. If they're not eating much, they may simply not be making any waste at this point.
The BML should have rep-cal in it, so adding more might make it tendt o be kind of bitter which may contribute to them not eating it. I had to start giving them straight replacement formula for the first couple of days and then wean them onto the joey BML.
For heat, I put a heating pad on medium under one half of the incubator so they could move as they wanted to to cool off. They did spend most of their time sleeping in the beginning.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry