If two couples are inviting the same guests, chances are they know each other and thus know if another couple is planning on marrying anytime soon. Besides, unless you belong to the same FC, most/all of your guests won't be shared. And if you belong to the same FC, you'd have heard in advance if someone else was getting married...
Honestly, I think some people would be ok with the possibility of losing a few guests, if it means not having to wait weeks/months to get a slot - which given how few slots are available, the wait time for a lot of people WILL be that long.
