If the non-riding child doesn't meet the height requirements, then yes, you will get a rider swap that lets the waiting adult ride and bring one additional guest with them. (3 ILL) Let the CM know before you enter the LL to add the RS, then the waiting party can wait elsewhere for the first party.
If the non riding child does meet the height requirements, then speak with the CM as to how best achieve a rider swap. Most times you will switch at the exit, but this varies from attraction to attraction and it's up to the CMs discretion as to the additional guest. (3 ILL)
If the non-rider meets the height requirement and you want to guarantee that neither parent rides alone then purchase the 4 ILL's. This way you can just switch bands to have the riding child re-ride with the 2nd parent using the non rider's MB (or keycard).