Actually, some of the baby swap passes state you can bring 3 guests with you on the 2nd ride. (Soarin' and Splash Mountain did this)
What happened on Splash Mtn is kind of hard to explain I guess. DH and DS had fastpasses since the line was over 45 minutes long. At SM, you can only get 1 fastpass per person, like many of the rides (I know in Epcot, they were testing giving out 3-4 fastpasses for different rides at the same time) When DS and DH went to the fastpass line, I asked the CM if I could get a Baby Swap pass (which is basically a fast pass for up to 3-4 people). So when DH and DS got off, DS and I used the baby swap pass right away and avoided the 45 minute standby line again. If we didn't use babyswap, DS would have needed to get another fastpass with me and that would have been another 1 1/2 hr wait for fastpass return if we wanted to ride.
BTW, different rides have different baby swap policies. On Space Mtn, the CM told me I would have to wait on line with the baby and DH & DS in order to do Baby Swap (the "old fashioned" way

) So I would have to wait at the boarding area with DD until DH and DS got off the ride and then I hand the baby to DH and then I could go on. The wait time wasn't too bad but since DD was really cranky, I decided to skip it this time.