As long as they can see each other. It’s like doing intros all over again giving them time to get use to Each other and giving the younger one more time to grow up.
Put them side by side for a coulpe weeks where they can interact but not touch. I have 4 Runner females who at 8 weeks old and the older group of Runners will chase them away. They did spend 2 weeks in side a dog X pen so they could meet the older ones so these older ones know these girls are...