Trust your gut and take your time. I’ve been in that spot, it’s flattering but confusing. Focus on who respects you, makes you feel safe, and aligns with your values. Don’t rush just to please anyone.
The truth is that one has to choose just one of the guys. I would examine the two guys and see which one aligns with my values and vision, and stick with him. I would politely tell the other guy it can't work and the reasons it won't work.
I think this should be something easy to deal with, the fact that you are already in a relationship should set you free already and the other guy needs to understand that too, else he's just desperate or obsessed with you.