It seems like you jumped into adopting this new dog a little too fast. I would not muzzle eather dog. but if things get heated simply tell them both to lay down and stay.
Be sure to keep them far enough apart (with empty space in between) that they can't snap at each other.We took them for walks but did not think to have them walking beside each other i was just so nervous that chester would snap at toby but we will have to try that tonight.