Welcome to the site!
There are a lot of stories around here about how people told their loved ones about polyamory. Some things you might find handy are the threads tagged "coming out
" and the ones tagged "mono/poly
" (assuming one or both men are mono).
I think one of the important things is just to try to be calm and understand that a freaking out period is pretty normal from people who expect monogamy and then find out their loved one is actually poly. If the guys know about each other, then it might not be as surprising to them as it is to others, but they still may need a significant amount of time to adjust to the idea.
I do wonder how they know about each other yet don't know that you spend time with the other? Is it just that they don't know how MUCH time you spend with the other or does one or both of them think that while you used to be seeing someone else, you are just with him now?
How long have you been with each one? Has it been long enough that you would consider cohabitation? I would definitely think that would need to wait and wouldn't be something you would bring up right of the bat. I mean, accepting your girlfriend is polyamorous and in love with another man as well as you is a huge step, trying to accept that she actually wants to move both of you in is something else altogether.