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 how they're feeling through all of it, and try to make everyone comfortable.
and this is just how i do things, but if my primary doesn't feel comfortable about something i do with a sweetie, or if he wants me to end it for one reason or another, we discuss it in depth...and, occasionally, i'll end it if needed. for me, my primary is my main focus, and if he puts the kibosh on me having other partners, i'll do it for him.