One thing about the Rider Swap passes is they can be used any time that day. This can make for extra rides of a favorite if you're willing to commit the time. EX: FP Space for 9:40-10:40, wife rides at 9:40 and asks for RS pass (husband and baby need to be there for CM to see) then when the wife gets off the husband could use his FP and ask for a RS pass too (again wife and baby should be there). Then they can go off and do whatever for hours and come back to Space and use the RS passes when they're ready. They do need to be used that day, and are now coded/stamped more like a FP (they used to be colored cards). This is only a bonus if you really want to ride multiple times, keep in mind that this takes a good chunk of time as the RS passes typically send you to the FP line and there is still a wait, plus the ride time itself. So if the rider waits 8 minutes to ride and the ride is 5 minutes long. You're taking about 15 minutes per ride X4 = an hour spent riding Space Mountain. You'd need to decide if it's really worth it to you.
You could use just one FP as mentioned in the original post and get a swap pass for the second rider. Then get a FP for a different ride. The distribution of the swap passes is a bit wonky and is not streamlined per ride. Some rides give them to the non-rider at the entrance after seeing the baby, other rides give you one of those black necklaces and tell you to give it to a CM further up to get your pass. And BTMR, is the worst as you are supposed to ask at the front of the ride and it's easy to forget, plus it back up the loading a little.