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?