i've never used it myself, but it certainly does exist - it is mentioned in the video guide in all the disney hotels. as far as i know, you queue as normal, then when you get to the front, one of you goes on while the other watches the baby/child. Then when your partner has came off, you (who, dependent on the ride, are either standing at the entrance or exit) swap the baby over and you can go on the ride.
Pretty vague, but as far as I know there is no 'baby swap' tickets issued or anything like that.