Depends on whether you are talking about Rider Swap or Rider Switch....two different programs.
Rider Swap is when everyone waits through the line and one person either does not want to or is too small to ride. One rider rides, then the other boards after the first returns and takes the reluctant or smaller child.
Rider Switch is when a person is unable to meet the ride requirements and cannot be left unattended due to age, usually. The CM will measure the child to make sure they do not meet the requirements and then will issue a paper pass that entitles the bearer to return to the ride through the FP line after the first has ridden.
Either way, one guest can return with you to ride.