need help navigating

my partner #1 and i have been ENM for almost 10 years with a few breaks here and there. we have mostly seen couples together but recently we met a single woman and it’s been exciting to each see her alone and together. over the last month i’ve been thinking more and more about a possible triad or girlfriend situation for myself. partner #2 would be interested in it and my partner #1 has made comments about it being fun. i’m having trouble navigating how to talk to my partner about the situation in a serious way. i’m good with whatever we decide.

i guess another question is what differentiates a girlfriend from your main partner? i have some ideas and i know every situation is different.

sorry if some of the terminology is a bit off