Well, your "discovery" is somewhat classic in that you've discovered the possibility of having a real heart connection with more than one person. And it sounds pretty classic too that they both have varying attributes you are attracted to that are unique.
But here's the one caution I'd add - one you probably have in your mind anyway. Your secondary (currently), being a single Mom with a 10 yr old daughter is likely to have needs, desires, and an agenda completely outside the scope of debating poly/mono lovestyles. While your primary might or might not be comfortable switching roles and becoming the secondary - even if she were and YOU were, out of practical reasons (money, stability, insurance etc), from what you wrote it seems there would be a huge question of your current secondary being comfortable with any aspects of polyamory.
So you are now put into a "principled" position of opening up with honesty as Catfish rightly suggested if you believe that in concept and reality, poly is going to be a requirement for your happiness - whether it's with these two particular women or not.
You can just acknowledge that you've seen the proof positive of the possibility but are not quite up to the reality of it. Which means you have to choose. But even that is not letting you off the hook because someone will be hurt including yourself.
I would try to get a discussion started with your secondary if you haven't already about the REAL advantages for everyone involved. Based on that conversation & reaction, you'll have some decisions to make.
Good luck ! I hope everyone can detach & think rationally.