Sj is correct
females can hold a pregnancy in stasis for up to a year, which is also why it was once thought that males sperm was still capable of producing joeys up to two weeks after a neuter.
IF the joeys aren't pulled by the mom, she will need the help of the other female since Dad is not there to play his role in raising the joeys. However, if the girls start having issues I would separate them but not until it was necessary to do so. They may do just fine, but the relationship is pretty new and things can change quickly even with long time cage mates.
There are many reasons joeys may be pulled or rejected and the potential for it with first time parents is high even in ideal conditions. So prepare yourself for worst case scenario which is losing the joeys.
If you are following one of the widely used diets give them an extra portion of the staple, Mom needs the extra nutrients while she's nursing, which they are already doing. Keep up with the extra portion even after the joeys come out of pouch, the girls will teach them where the food is and when mom no longer eats the extra, the babies will begin to.
Most people do not need it but get a joey rejection kit to have on hand in case you do.
Study up on signs of rejection, growth, ect... http://www.suzsugargliders.com/helpforrejectedjoeys.htm
This site has excellent information on joeys, rejection, weights, care etc...