I could go for the 'Twisted Romeo and Juliet' idea, buuuuuuut...
I don't think I'd be very interested unless we could tweak the idea juuuuust a little bit.
Which is to say, I'd be more than willing to play both Demon princes if it turns out that : The reason for the elder prince's attitude is that he's in love/lust with his younger brother, and specifically requested the marriage to prevent said brother from winding up with someone else.
Threesomes = love, and most of your Ons are also mine. Reply if you like, huh?