There is so much wrong herein three lines I do not know where to start. So I guess at the beginning.

when I backup these huge folders (Local and Roaming) by copying them

Why? The local storage does not contain anything but caches and temporary storage. The version downloaded from the Microsoft store is somewhat different, but fortunately few folk use it.

Next, what are you really trying to do. Thunderbird may or may not have files names with the email account in the profile folder. Mostly I do not have anything but server names in my storage folder names and not atall in the storage files.

If you want the specific folder an account uses for it's storage, look in account setting at the bottom of the server settings for the local directory.

If you actually want backups of the profile, I suggest you use this add-on to make them. https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools-ng/?src=ss

Matt said Why? The local storage does not contain anything but caches and temporary storage. Carey says: you are an expert and you know these things. How was I supposed to know that? Matt said

Thunderbird may or may not have files names with the email account in the profile folder.  

Carey says: how do I find out if the second account is backed up then? I don't have a spare Thunderbird installation to restore the backup to to see the answer. The folder my 2nd account uses is the same as for the main account. Should I just trust that with no evidence? Matt said: Why? Carey says: because Thunderbird instructions for moving to a new PC say copy the whole Profiles folders there. To me that is proof that copying them to a backup location is guaranteed to preserve them. Disk storage and transfer speed are very fast now and the copy duration is acceptable to me. What could be simpler?