I think it is very natural, they are communicating.
What to do? Depends- for quite time or for concern? If for quite time, I would suggest managment techniques. 1) Get two Kongs, stuff them w/ whatever their diet consists of and let them have at it, 2) Walks- excercise means tired dogs, less barking. If for concern, unless they are showing signs of aggression- aggressive growling, showing teeth, then they are being dogs- let them. Try and see it from their world and a lot of our concerns vanish.
Can you identify the "type" of barking (are they aggressive / territorial, fearful or wagging their tails inviting each other to play).
Depending on the type of bark then you could act. If its aggressive or territorial don´t keep them together, if its out of fear watch out since it could trigger fear biting, if they are inviting each other to play, I´d let them rump until they tire out