Most Helpful Guy
good troll, but I will answer anyway in true spirit of the troll.
find out if they would be willing to fight for your honour first - ask them both how much they care for you and if they would ever be willing to fight for you.
if they say no, try to subtly instill values of decorum over time. tell them how much you admire certain values they have and how you would simply love a man that would be willing to fight for you in honour.
when you are confident both men would be willing to fight for you, this is when you say that you have another potential suitor. but you do not mention that you have already been dating the other guy behind his back. you just say that he is someone that has shown a lot of interest in you and is willing to fight for your honour. would the other man now be willing to fight for your honour as well? make it clear that a man who is not willing to fight for your honour is not worth your time.
Most Helpful Girl
I highly doubt either of them will care enough to physically fight each other for you. More than likely they will both just go "we're supposed to fight over you? Meh, not worth it. I can throw a rock into a crowd and find another mental patient exactly like you."