RE: an unusual question
type of dog matters also. the more domesticated breeds can stand a little more interaction with other dogs at a younger age. those breeds, especially hunting breeds and wolf-like breeds, should be kept seperately as they tend to bond with dogs MUCH more easily than with humans. I have laikas, ancient russian hunting dog. these absolutely must be kept seperate in order to "train". once they have bonded to you, the trainor, they will view you as the lead dog and never question your authority. if you allow another dog to take this "lead dog" role, then a laika will never view you as an authority and will be very unruley. most real hunting dogs will behave similiarly---maybe not as pronounced.
poodles may be trained together. what's a poodle?!?!?