When I had Buddy and Obi I left them together. (Buddy was a "pitbull" so add me to the irresponsible list. Looks like I'm in good company.) But Buddy was a saint and Obi is an asshole but I knew my dogs.
If I get a pup, they wont be left together because, as I said above, Obi is an ass. So unless the pup is Buddy reincarnated, it just wouldn't be wise.