persevere. seperate them for now, have their cages side by side and let them have play time together.. the older one will assume dominance and will have to make sure that the younger one knows its place. it always looks and sounds worse than it is as long as no blood (scratches no included) is drawn they will most likely eventually get on together.
You need to get them out together for at least 20 mins a day to let them get used to being together, don't give the younger one anywhere to hide (but on you). if you show the older rat that you accept the younger rat but don't admonish or ignore her then she should follow your lead.
it might just take abit of time.
once they can play together fine, try sticking them in one cage (after you've cleaned it out) and letting them play in there under your superivsion (door open), this might also take a bit of time, but it might be instant acceptance. you never know with rats they're funny buggers like that.
