My wife has a 910xt and occasionally I use it too (mainly for swimming since I already have another GPS watch for running/biking). Before Garmin Express we were able to upload our activities to our separate Garmin Connect accounts (used Ant Agent to get all activities from watch to computer, then used Upload function on GC to just get the desired activities per account). Pretty straightforward process. Details here:
https://forums.garmin.com/showthread.php?32993-how-to-use-one-910xt-for-multiple-users
But now with Garmin Express when the activities are loaded from the watch they are all going automatically to the linked GC account. I get Garmin wants you to buy a device per user but that is expensive. Anybody solve this issue? I don't remember if it's possible to change the linked GC account without re-pairing the device. But even that wouldn't help a whole lot since there is no way to pick & choose specific activities. I think the only way to do this now is on my GC account to use the Import function and grab the FIT file I want (one at a time). On her account, she would have to delete the activity once it was uploaded to GC (if she only wanted to keep her activities on her account).