Help merging groups.

I have two groups of two that I DM and we’re merging them into a single group of 4 but I can’t work out how to do it in a way that makes sense to the story….

They’re both low level only a couple sessions in, one is a pair, unknown to each other, both from the underdark and they’ve just broken out of captivity from a bounty hunter sent to capture them. The other is a pair that woke up finding themselves in a strange experiment by a mindflayer where he was capturing their dreams.

Any ideas or inspiration would be welcome!! I can’t think of any way to do it which doesn’t feel forced etc.