Hi Everyone,
I just ran into the same "question" (consciously not saying issue ;) and was puzzled for a second too, but it all makes sense.
To repeat:
We come from the "same place" I guess, we're logged in to the private PC, using a Microsoft Account, then trying to add within the OneDrive settings another account, for example a work account, the result would be the same in case the sequence is switched, because it's contradicting.
Why:
In the default settings the additional OneDrive will be added to your user folder, as you are logged on to the system and so it is within your user's folder structure & security context, unless you specified a custom location. (That wouldn't change the result btw..)
On your initial OneDrive from the Microsoft Account, Backup & Sync is turned on (oh I love this feature! ^^) and it backs up the local (private!) content of your user account to your OneDrive - Personal.
Now the message that got us all here..
..just wants to make you aware, that your second/work OneDrive won't try to synchronize the folders which are already handled by your personal OneDrive. - True, it looks a little scary, but it doesn't mean that when you create & add files to your OneDrive - <WorkAccount>, that they are not synchronized from your PC to the Cloud.
Because the message refers only to the "Backup of your PC", which takes care that all the "Standard Folders" from your User are uploaded to your OneDrive - personal.
https://support.microsoft.com/en-us/office/back-up-your-folders-with-onedrive-d61a7930-a6fb-4b95-b28a-6552e77c3057?ui=en-us&rs=en-gb&ad=gb
and safely stored there, in your private Micrsoft Account's context, so if you lose, re-setup, or start using a new PC, it's all there! <3
[Please note: Only as long as ALL your data is stored within the standard locations]
To Summarize:
For the Backup to work, the presented Desktop of your private Microsoft user account is residing within your OneDrive - personal, so it gets synchronized by the agent vs. the original/default location: 'C:\Users[Username]\Desktop'
-> That's the reason, why your OneDrive - <WorkAccount> cannot overtake the same task, as there is only one Desktop existing for your private user and that's living within your OneDrive - Personal! ;)
Hope that helps!
cheers,
Alex
PS: Just imagine if your OneDrive - <WorkAccount> would start uploading all the content of your private User's folders [Desktop, Documents, Music, Pictures, and Videos] into your WorkAccount and vice versa, what a mess it would be!