First check out what rims are on the bike. I can't assume they are factory original to the bike. These tandems used the Schwinn Tubular S-7 rims and that should be stamped on the rims in the center along the spoke line. These had 105 spokes that are a heavier gauge than the regular bikes. If the wheel sets are original to the bike then you'll know what your tire options will be and your front brake caliper.
If I remember correctly, my 69 tandem had this front caliper. Type MS 3.1. I parted out the bike years ago and the caliper is long gone.
I have two of the se - one is a "Bicycle Built For Two", The other is a Twinn. First is the '63 "trellis" model BBFT: (photo is from ad where I bought it! It's currently hanging on the wall in the storage shed')
Second is the '64 Twinn:
The rear rack has been adjusted since this photo qas taken a few years ago, and it looks "Mo Betta" now
I found a front fender, two good Schwinn Westwind tires with tubes, front hand brake complete, and 4 Blue Sparkle hand grips. Paid more than I wanted too, and was supposed to get two rim spoke bands with the deal but they were not in the package.
Today I cleaned up the rust on the front rim, disassembled the axle and repacked the bearings, then reassembled it with a tire and tube.
The front rim seems to be too small to seat the tire bead. Upon cleaning the rim and hub it is not an original S-7 as I had hoped. The rim is stamped "CMC RIM" and the hub is stamped JOU YU 3 84. How do you determine the rims size...its like this in possibly "24" or 25" rather than 26".
Looks like all the wheel sets were changed out. A 1964 model would have the single center knurling with center stamped identification and a Red band hub. All tandems had heavy duty spokes. 1964 was also the first year for the Twinn.