I favor hands together – especially w/ runners on base – for the simple fact that it allows you to make a quick throw over to first base. It’s also easier to get a good grip on the ball and not search for it or tip off what you’re throwing.
But, I see a lot of pro pitchers start hands separate. What do you do/teach?
Together. If you drop the ball with runners on, it’s a balk.
Hands together—and use that glove to conceal your grip so the opposition can’t see it. Also, I don’t believe in “showing the ball” to anyone, second base, outfield, what have you, for the same reason. And since as a sidearmer I used the crossfire practically all the time, I could release the ball at the last second.