All of our buses have two securement locations. The majority of the time, all works well, and goes smoothly. Of course, since Murphy and his law vacation here in Florida all the time, there's always the chance of a snafu. (ie: during a park exit when everyone is leaving at once)
In the event that one (or both) position is already taken, you may always fold up a wheelchair, and have that person take a regular seat. The folded wheelchair(s) can go anywhere on the bus, just as long as someone can hold onto it.