Help, I'm confused and want to plan appropriately!

When it comes to fast pass plus and child swapping, do both adults need a fast pass plus in order to utilize the fastpass line and obtain a child swap?

aka:
Only Dad has a fast pass for Splash Mountain.
Dad gets in fast pass line, Mom gets a child swap.
Dad rides.
Mom uses child swap to hop into fast pass line with child swap pass.

I ask only because there is conflicting information. Two pages on the disney mom's panel website (an official disney site) say you only need a fast pass for the original rider. But on our last trip a cast member at one ride said we both needed the fast pass. Other cast members said no, only the original riders. I'm so confused.

I'm by no means trying to skirt rules, I just want to plan my fast passes appropriately.
Thanks!