You might think that, but it's not a correct assumption. The sync chain serves one purpose and that is to transmit the front rider's power to the rear bottom bracket spindle and it does so on conventional tandems through a 1:1 direct drive system using, in most cases, relatively large (34t - 48t) chain wheels or, as they're more commonly referred to, timing rings. So, you have only one rider's energy going into the system.

Now, to be fair, your daVinci is an exception in that your sync chain is subjected to higher loading than a conventional tandem without the small BMX freewheel sprockets used by your Independent Coasting System's jackshaft. However, even so, the loading is still not all that much more severe than a single speed / single bike since you don't ever end up working against a stoker with an inefficient pedal stroke.

Now, let's look at the drive chain: it must handle the power of both riders on a variable gear system that uses sprockets with as few as 11 teeth and, for triples, granny-gear ratios on 27t - 34t sprockets, and that must also deal with side loading and wear from the front and rear derailling process.

As for "high spots" stressing the chain, you'd have to use a pretty large lever to tighten the eccentric enough to get anything close the amount of loading on the chain as it sees when it's under load from normal pedal input by the captain, never mind how much that chain loads up when the captain is standing on the pedals and climbing or sprinting at a maximum effort.

Yes, the high spots are annoying and very disconcerting when you spin a tandems drive train without any kind of a load on it but that's abo