There is no need to separate until they are 3 weeks old at which time you will also be able to weaned them from their mom, separating now will mean separating the mothers that are lactating as well one will have 8 while the other will have 5, keeping everything communal while nursing makes it easier on the mothers as they take turns and the difference in age does not require to separate them from one another either.

Again you are trying to complicate things instead of keeping them simple.

As for sexing http://www.afrma.org/sexing101.htm