Who's the manufacturer and what year was this tandem bike manufactured?
It's a Schwinn Twinn. The serial number will be located on one of the rear dropouts, if it's not on either side of the head tube. There are a couple websites that will help you determine the date of manufacture. Judging from the type of tires fitted to the rim, and the shape of the rim, I surmise that the bike has had both wheels replaced at some point, as the bike no longer has S-7 rims and tires, which would have been stock on a Schwinn.