What, you mean you all don't have a "fall back" couple?
We Have had this happen, have even done it ourselves. Doesn't mean the couple wouldn't have chose you first at all. Often times, in our case anyway, we will get to the club and start a conversation with someone and it seems like it might lead to sex. Only to find out after spending an hour or two with them that they aren't compatible with us for some reason. At that point we move on, and if we are really in the mood to play we might see that Joe and Jane, who we have had fun with before are available so we approach them.
So, I think that it doesn't really matter.