There will always be a dominant one but you might not notice wny signs at first. When they approach 12 - 16 weeks one may start to chase the other now and then. They will still snuggle together, then one day the dominant one will start a fight. It's really difficult to pinpoint an actual date, it's usually around the 14 week stage. You may be lucky and get 2 boys who don't display any signs at all, I have 2, but this is the exception rather than the rule.