Ah, it takes time... You need a split cage.

Read this:

http://www.twinsqueaks.com/split-tank.html

http://www.peteducation.com/article...&articleid=1622

http://www.gerbilbreeding.com/intro.htm

http://www.petermaas.nl/gerbils/introducing.htm

6 months is still young, so you can try it. Ask your mom to help you build the split part of the tank.