Can I lend my Edge 820 to another person during week days?

Not at all easily. The sensors are the easy part. Pair the sensors and the 820 will connect to whichever are active when it's powered on. The 820 can only be paired with one phone, so you'd have to forget the pairing on the 820 and pair it with the other phone every time you made the change. The other issue is connecting with the phone app and GC. The 820 can only be associated with one account, so you'd have to remove it from the one account and associate it with the other account each time you made the change. And, finally, all activities stored on the 820, both yours and your wife's, will be uploaded to whichever account the 820 happens to be associated with, so you will see all the activities in both accounts unless you first delete them before switching accounts.

This is my understanding as of the last time I looked at this. Perhaps some else knows differently, or there have been changes made that make this easier.

Not at all easily. The sensors are the easy part. Pair the sensors and the 820 will connect to whichever are active when it's powered on. The 820 can only be paired with one phone, so you'd have to forget the pairing on the 820 and pair it with the other phone every time you made the change. The other issue is connecting with the phone app and GC. The 820 can only be associated with one account, so you'd have to remove it from the one account and associate it with the other account each time you made the change. And, finally, all activities stored on the 820, both yours and your wife's, will be uploaded to whichever account the 820 happens to be associated with, so you will see all the activities in both accounts unless you first delete them before switching accounts.

This is my understanding as of the last time I looked at this. Perhaps some else knows differently, or there have been changes made that make this easier.

Are you sure it's that difficult? I think it could be easily done. I manually upload my activities (no auto uploading), so I can easily upload whichever activity to whichever Garmin Connect account I want. You say that the 820 can only be associated with one account. I don't think that's the case. I know for a fact that I have uploaded an activity from my girlfriend's Garmin 820 that she did to my own Garmin Connect account when I wanted to see her lap times for a certain event. So I think it's pretty easy to do this. Just remove the activity .fit file from the device and save it locally after each activity and then upload it to wherever you want. Then there won't be any confusion about whose activity it was.

I think we're in agreement except for the characterization of easy vs not easy. Edge devices, GC and GE seem to be architected to work with a single user, but it may be possible to concoct some workarounds. Manually copying, uploading and deleting files for each user will certainly work. Granted it's not overly difficult, but it negates some of the automated features, things like automatic uploads, notifications, Livetrack, etc. I haven't experimented with these in quite a while so it may be possible to share a device and pick and choose among these features to get it to work acceptably. For example, it might be possible to pair a single device with multiple instances of the GC app, each of which is linked to a specific account, or use one account and disable auto upload, etc..

Based on my experience with my Vivoactive, what you could do is point GCM to one account and Express to another. However, keep in mind that your wife isn't a strapping young male which might or might not matter when it comes to an Edge. Also, at some point lines are bound to get crossed

yes, you can. just need to delete and pair to your/ your wife's phone everytime you switch. remember delete all the history file before switch.

I do this for around half a year with my wife, then i buy another edge because we seems the switch quite more often than before.