Can you wear two reflectors, one on helmet and one on backpack, or will they somehow confuse/cancel each other out?

We wear two when in the outback and they do not cancel one another. Actually , after research, we read that if your backpack or helmet or whatever is ripped off, the other hopefully stays with the person. We went with the helmet and the belt option. Hard to lose a belt.

Is this useful for hikers? would it help sar locate you if you got lost while out on a trail in a non-snow environment?

Yes this can be used in summer and winter Please check here if there is a sar operating where your travelling to https://www.snowsafe.co.uk/list-resorts-use-recco/

Does this work on motorcycle helmets?

I guess if you wear a motorcycle helmet while skiing it would work otherwise I doubt it since it is primarily meant for buried in the snow rescue recovery and most people don't ride motorcycles and deep snow