* One account required me to log in with a username, and was associated with my main email address. * The other account required me to log in with my main email address.
In a way, they were both related to the same email, but different accounts. This shouldn't even have been possible, but it seems that one was an old MS account, and the other was an old Skype account. My roster ended up being split half and half between the two.
Very frustrating - fortunately I don't use Skype regularly but it's always an exercise when somebody asks for my username.
The steps I found to unlink within the XBox UI didn't work, futher research now leads me to the following discussion which I am about to try: https://answers.microsoft.com/en-us/outlook_com/forum/oemail...
Edit: it looks like this was possible in the past (basically deleting the Skype account?), but not anymore.
[1] https://support.microsoft.com/en-us/help/12412/microsoft-acc...