If you use single rider, chances are you will be split up from your companion. I was with my DS13 at DL earlier this month and we got a Splash FastPasses so we could ride together. And we did. Then later that day we wanted to ride again if we could do single rider. I have never done single rider before, as I am usually with my whole family. In any case, we did it and were third and fourth in the single rider line. After about five minutes they needed two people quickly, and told us to rush up to the front log. So in the end we got to ride together. Usually when they need two people they call out for two from the standby line.
As noted by others, only Splash Mtn at DL. At DCA Soarin has it but apparently the seating is such that they do not need singles very often and thus the single rider line is not very efficient. They have one at GRR and at California Screamin. I think they also have one on Mailboomer.