Logging in as user 2, screws an already open session as user 1, same Firefox.

Easy. Log out of your wife's account before you open yours.

This is not a behaviour specific to Garmin. Same also happens with Strava and SportTracks.mobi. It is not, well I have never been able to, possible to have two accounts open at the same time for the same online service.

I suspect this may might well be the same for other online services too.

This is going to happen for any website that has user specific information and multiple pages or views. When you log in the browser is sent a token or cookie that identifies the user and the source of the information. If you log in as a different user then that token is changed.

You might want to try using a different browser, such as chrome, or the 2nd account.

Yes, I have already experienced that with MyFitnessPal but to me site developers
should try to find a way to let a group of people (family, fitness club, whoever)
to have more than one account opened at the same time, for many reasons,
primarily for comparing data or even just for cross checking data between
different devices registered on different accounts.
If the server can manage two accounts opened from two different browser,
I think they could do it even from two different tabs of the same browser.
But maybe it's a browser limitation.

In any case and above all, I shouldn't be able to see other's people data without
knowing their login name and pw.
As it is now, if anybody else open his/her own account on the same PC
or tablet I was working on, their data will appear also in the tab where there was mine
and they wouldn't know it.

Not too fare in terms of privacy rules.

Anyway, thanks for the clarification.

What you are asking for is a bit unrealistic this behaviour is common to thousands of web based applications - if you open separate tabs within one browsing session then you should accept that

If you are worried about privacy between different people using one PC then set up different users and control access appropriately

For two accounts, I just use firefox and chrome, with one account on each. Otherwise, I can see where things like cookies would get confused.

I've not tried it, but in chrome, there is "incognito" where things might be kept separate.

...
While you can't do this in a separate tab you can open a new private window with the same browser. I think that will work, but not sure.

I tried and opened another instance of Firefox but the behaviour is the same.

What you are asking for is a bit unrealistic this behaviour is common to thousands of web based applications

OK, but that doesn't mean that it is good or that it can't be better.
To me, the best thing, if it is possible, would be to have a setting where you can choose
if you want to manage one or more than one account at a time, even on the same PC.

Another thing that make me think this is not right, is the fact that in Garmin Express
you can deal with many devices all at once. But if in GE you click on the link to GC, it will always open the
default browser so you can't select one device and open GC and then select the other and do the same,
because you'll end up with two tabs or two windows of the same browser hence with twice the data of the
last account you opened. So in GE they should put at least three links: "open GC with xxx browser"
and list the available browser.

But, as we apparently can't solve this problem, I suggest we all go and spend our time doing something else.

Thanks to everybody for the support.

fabio