There shouldn't be water near the heat lamp. The heat will create a great climate for bacteria, so you should place both away from the lamp.
Make sure they know how to use it - maybe you can get them to do it by showing it with your finger.
Depending on what you feed them, oyster shell isn't...
The cage I keep mother and daughter in is about 8 square feet. A female group might be just fine in 4 square feet - but prepare for a situation where they are not.
Actually I dare say I have more success keeping two males together than two females.. If the males can be kept together at all (are not constantly chasing each other - which they usually aren't if they were raised together and there are no females around) they usually get along with few...