Generally speaking, dogs of the same gender- especially an unaltered male- will not tolerate each other reliably. You can make it work, but it can be incredibly tough. Neuter the new pup and your chances of them getting along increases, but your are going to have your work cut out for you.
Go to neutral territory and have a friend help you. Walk towards eachother (not directly head on, but in an arc) with dogs on leash from a distance of about 100' away. Keep coming towards each other in an arc at a decent pace and watch for any type of reaction- there should not be any. Not even an eye twitch if you are doigit right. Keep "arcing" towards and around each other. If you get even a small reaction you've gone too far, end for the day. You are done. It's very tough to explain this whole process in one email. The big poitns to remember are to arc them and not have them challenge each other. Let them succeed. Stay on neutral territory. Praise lavishly for not reacting, and do nothing if there is. Dont' let them lock hard stares. That's a challenge.
Good luck and there's few books on the subject- one I know is called 'Feeling outnumbered' that can adress this in more detail.
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Chelle and the rest of the crew including, but not limited to Kita and Taiko (the shiba inu wrestle maniacs), Adi (reserved and dignified tabby cat), and all 28 reptiles