i'm not sure why apple haven't done this already, to be honest, but I can imagine they will eventually. Perhaps they think that encouraging people to get 2 or 3 different ones takes away from being "their most personal device ever" when you can leave one or more on the side at home? (maybe they're hedging their bets for now, in case they work out how to do a round one the right way? if there's something apple hasn't perfected yet, they'd rather you think its a crap idea and sell you on it later, than rush out something they messed up, IMO)
anyway... i could see it working if you kept them all on charge (or at least charged up today's and tomorrows choices before bed?) and perhaps they're waiting to see if leaving one on the charger without taking it off would diminish battery life greatly?
anyway, imagine this:
watches are all on charge overnight (on the side, or wherever). phone is plugged in. all is kept synced to the phone while i sleep. (if a watch hasn't been used for a few days, maybe there's some photos to add, playlists to sync, workout data to sync over etc etc).
I wake up and choose my watch. I put it on my wrist. Wrist detection tells the phone I've put on my space black today. Phone unlocked with touch id, and so the space black on my wrist is active for the day. other watches sit at home, idle.
I wear it for the day, but that night I really want to wear my milanese loop. I drop my SB watch on the charger. Pick up the SS. The watch going on the charger tells the phone it MAY become inactive (because I may decide to put that back on after charging, no sense disconnecting); the SS going on my wrist tells the phone, time to activate me! Watch is 100% charged, so doesn't mind a little syncing on my wrist of the important things (activity, mostly - other things as above could have updated during the day over wifi anyway?)
And... repeat.
I think post os2 and the watch becoming more independent, this kind of thing really should be possible. What would happen if someone else put on your spare watch? Not sure, maybe a popup telling them your other watch is already active and flipping it into power reserve mode, so it just tells the time? (Great if you want to let your kid feel important by wearing your apple watch, I guess. From what i've seen, that'd definitely get used out here in China)
thoughts?