To do another Master's, already holding a PhD If a STEM PhD holder applies for a Master's in another STEM major, would this be an advantage or disadvantage in the eye of the admission committee?
Surely you're joking Mr. Feynman!

Sorry, but you walked into that joke. Actually, such an application would occur so infrequently that it might result in a few chuckles. But I don't think it would have a lot of effect. But certainly not a big advantage. 

In fact, people, including myself, will wonder why a person would bother. After all, the applicant can study and learn effectively and evidenced by the doctorate, so what is the need of a formal program. It might be a disadvantage in some places where the available slots are limited and people would feel they should be given to someone who needs the training more. 

On the other hand, if a lab needs particular skills that you have, they might be interested, just for that. 

Thus, it is impossible to say in general as local needs/beliefs will dominate such a situation.