If a boy says “yes” he should give the girl a week to answer, if she doesn’t answer after a week, there is no reason why he can’t give a “yes” to another girl and take it from there. If the second girl gets back to him first there is no reason why he shouldn’t go out with her first.
Hashem is in charge, we are only players in HIS game. If it is meant to be that the first girl is the one for him then the date with the second girl will not work out. On the other hand, if it does that is what was bashert for them. The point is that you have to be mentchlich but not stupid. You can’t wait around for every answer because it could be a “yes” or a “no”. So after waiting a week which is fair and you don’t get an answer move on to the next one.