To clarify, it sounds like you may want to divorce your wife, remain co-parents, and develop a monogamous relationship with your girlfriend?
Are there reasons when the relationship with your girlfriend became more serious why this was not an option? While you are not in a sexual/romantic relationship with your wife, you can be considered non-monogamous because of the deep emotional connection parenting and being best friends have forged between you and your partner.
It would not be a bad idea to meet your girlfriend's interest but just meeting her is not likely to help. It sounds like you and your girlfriend have much to discuss about what each of you really want, and what that would look like, and how to get there.
What do you want? Do you know what she wants? On a deep level, beyond buzzwords like poly or monogamy, to what each of your hearts and minds truly wants from each other.