Yes, I'm aware that it is not recommended to wear a Garmin watch while using the ISM. I only discovered this after several days wearing both together and getting weird results. I eventually found the information on the ISM support pages, but why not include it in the ISM Owner's Manual? It might save some users time and frustration.
Also, surely it would not be beyond the capability of Garmin's engineers to make it possible to wear both devices at night and simply have the ISM data take precedence over watch data for the relevant metrics? I have a HRM-Dual heart-rate-monitor and there is no incompatibility wearing both this and my watch while running. If it can be done for the HRM, why not for the ISM?
Another thing that is less than optimal is that body-battery information is not recorded when wearing just the ISM and not the watch (as others have noted). Body-battery is important for me. The ISM seems to record all the info necessary for calculating body-battery, so would it not be possible to fix this? (I'm aware of a suggested work-around, but it does not work for me.)