Why are the OP and most posters measuring tire sizes in inches? What rims is this tandem running? What make and year of tandem? Because if this is an old tandem running hookless rims, then the answer will be totally different. There are no modern 27" wheels, only 700c or 622mm, and thus no modern tires.
If they're 700c, then the answer is certainly Conti 5000 32mm tires. If they're 27" than, as above, change the rims out to modern hooked rims, and run the Contis. Check with your bike shop to be sure that your brakes can be adjusted to suit the new rims. We've been running Conti tires in 32mm size, both 4000Iis and 5000 and like them very much. Quite flat-resistant which may be a bit counter-intuitive, but they are. I think it's the rubber compound more than anything.
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