it's pretty organic, really: be honest, talk things out, don't hide anything. don't go looking for drama and it shouldn't happen if everyone involved is an adult about it.
pitfalls are entirely up to the people involved. keep up the communication with your primary...make sure you know how he's feeling through all of it. ask him how much he wants to know of your doings with NewGuy, and discuss the same with NewGuy.
constantly touch base with both of them about...