I wanted to share with you my way of centering the chainrings on a tandem.
I know that the late Sheldon Brown has some wonderful web literature on this subject as well. I've read his methods and mine is slightly different and ended up working better for me.



With the chainrings loosened to hand tight (5 bolts on each chainring), find the "high spot" by spinning the crank; it's where the chain has the most tension. In that position, put a zip tie or a rope around both top and bottom of chain, bringing it together a bit to provide extra tension and force on the loosened chainrings (see photo). This will force the chainrings to come together as much as possible, reducing the high spot. DO NOT PUT EXCESSIVE FORCE ON THE CHAIN WITH ROPE/TIE BECAUSE THE MECHANICAL ADVANTAGE PUTS EXTREME FORCE ON CHAIN AND CHAINRINGS. Now tighten the chainrings, at least 3 of the 5 bolts on each. Remove the rope or zip tie and spin cranks.

If the high spot is still in the same location as when you had the chain tied, then you've done the best you can. If the high spot is somewhere else now (usually on the opposite side), you can continue fiddling by loosening only one chainring and using the technique above or a different technique.

In my case I was able to get the chainrings so perfect that the chain moves up and down about a millimeter and the chain droop is no more than 5 millimeters or so.

Sheldon mentions rotating the chainring to a different position (meaning, lining up the 5 chainring holes with the 5 spider holes differently), but in my case that was not needed. The rotation of chainring will work especially if the chainring itself is not round.