
Its definately best to wait until the first dog is an adult.
Before that, even with a good few months in between, they will just play as puppies do, bond together more strongly than with the owner.
Often two young dogs together DONT learn how to be sensible adult dogs, to them rough puppy play and no manners is acceptable so they will try this on older dogs. When a pup IS a small pup, adult dogs usually tolerate this and reprimand where needed. When an older puppy does it, a fight can ensue, the pup is not young enough to be granted leeway.
Wait till your bitch is approaching 2, has learned dog language and responds correctly to other dogs, then get a male puppy.
Em