This is the trick, though. They want to improve their system. By sharing with you they may have solicited some really helpful advice.

This has sounded ok for me too, but when I've thought a bit about it I've realized that the drives must be synchronized so that each one would "cancel" each others vibrations. The vibrations generated by the drives might have indeed the same amplitude and (spatial and temporal) frequency, but I doubt that they'll have the same phase offset. For this to happen they would need to be started at _exactly_ the same time which I don't think that happens in practice.

Do you have some pictures of those enclosures?

I don't have pictures. I can describe, though.

Two disks were mounted in a frame linearly, both screwed to the frame, with the power/sata connectors toward the middle of the frame, and one drive upside-down.

These cassettes were removable as a unit for hot-swap, and were inserted linearly into a half-deep 19" rack enclosure.

That the two drives were physically connected to the same frame, and removed and replaced as a unit would make it seem as if they would be started in phase. Now I'm not physicist, but I'm not 100% sure that's so important - if you have two gyroscopes contra-rotating and firmly connected, running at the same speed, surely they resist movement by sheer gyroscopic effect?

Unfortunately the SATA backplanes are only aligned one way, so they could only rotate a set of 5 drives at once.