I wouldn't go out and separate them straight away if they are getting along well, especially if you haven't had them long. If you can see they are interacting well together and seem happy together and are growing at the same rate (no obvious size difference), I see no reason to separate them. You do need to take some extra precautions with pairs though.
Make sure there is at least two of everything in the cage and no way for any hamsters to get cornered in any toys or houses. Levels can make them territorial, so remove any of those. Scatter feed to prevent food hogging. You do need to be generous with this. Avoid changing the cage set up unless it really is necessary. Keep an adequately sized back up cage in case you need to separate them at a moment's notice.
If you see that they are no longer happy to living together, separate them even if they aren't fighting. Better safe than sorry. If they do end up fighting, they may need to see a vet to treat injuries, so it helps to have one in place as a precaution.
As for your original question, unless you have a ton of experience with introducing hamsters and keeping them in pairs and have the financial capabilities to care for them, I really would not add to your original hamsters. They need to ideally be litter mates kept together since birth. You can get more hamsters and more cages though.
Good luck with your hamsters
!