I've been - openly, but they don't know each other - seeing two different guys, for about the same amount of time but not really seriously on either side. I want them to hopefully turn into actual relationships I'm just not sure how to bring about the conversation of "I want to be serious with you, but also him at the same time" and am scared that will send them both running. Any advice?
The great thing, and the terrifying thing, about relationships is that there is no set of magic words you can say to ensure someone else responds well. You should definitely have that conversation, and then if one or both guys goes running, well, that’s just what happened. You can’t prevent or control that. Hiding your feelings/desires from a partner for fear of scaring them off is miserable, and not something I ever recommend.
You’ve got to just come out and say “hey, we’ve been seeing each other for a while now, and I really like you! I’m interested in talking about how we see this relationship moving forward, how committed we feel, and what we both want! And that should include the fact that I’m also having the same feelings about, and conversations with, Orzaggo.” And then you two can talk about how he feels about non-monogamy, and about dating you, and about feelings, and all those super sticky but worthwhile conversational topics!
And if he runs, well, that’s information you have - that he isn’t someone who wants to date you as all of who you are, which is someone capable and desirous of dating multiple people. You would have found that out eventually; delaying this risky conversation doesn’t reduce the risk, it actually just raises the stakes. Best of luck!