I strongly suspect that we both have the same concept of chainline in mind, and a discussion at this level is not needed. It is true that wider stays typically go with wider hubs (or at least more dished hubs), and therefore wider chainlines, so there is some indirect reason to link the two measurements.
But, the jerk in me has to try to get the last word in. So, thanks for your patience with me.
The tandem you have has a wider chainline because of (I would guess) a wider hub. Not wider stays. Your own numbers show this. The difference of half of each stay width in your examples is 15mm. The difference of half of each spindle length in your examples is 5.5. If OLD had anything to do with chainline then this would suggest that one of bikes had a chainline that was about 10mm (15mm-5.5mm) off. I bet both are much closer to straight than that.
This is a bit oversimplified, since different cranks sit on those spindles in different ways; a triple compared to a double for example. And it is a bit presumptious of me to assume that one of the two bikes in your example has a straight chainline, which therefore means that the other would have to be off by 10mm if OLD was relevant; each could be off 5mm in opposite directions which could spread the difference acceptably between both bikes.
jim
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Last edited by jgedwa; 11-09-08 at 07:14 AM.