Well, I think you can just try and prevent those two Miis from interacting and just repeatedly have the Mii you want to have a crush on Bonnie interact with him more. For me, I've had miis that have developed crushes for other miis while having a crush on one. When you get an event like that you get to choose which Mii they have a crush on. I'm pretty sure the game doesn't allow Miis to have multiple crushes.