Timing chains last 3-5x as long as the derailler chain on a tandem. It does not matter what kind of chain you
use. 5-6spd chains are fine but 8-9spd are also fine as are 10 or 11 spd chains. The current timing chainwheels
on the bike are likely 3/32 and will be compatible with any chain from 5-11 spd. Single speed chains could also
be used but connectors are a bit sparse as they are mostly connected by driving pins in and out. If by rare chance
you do have 1/8" timing rings then you will need to use a 1/8" chain. Your tandem will have an eccentric in the
front bottom bracket that rotates to adjust tension.