They mature 8-12 weeks(depending birthday) will mate quickly when ready. Separating them will cause a depression however if they are brother and sister breeding will cause defects and health problems for the Joeys as well as the mother can start self mutilation. It's always better so keep same sex Sugies together. You have to decide between the two for I wouldn't be able to choose.
If you decide to separate them for the week or 2, a lot of love and attention is needed. My one started self mutilation only after 1 week of her sister dying.