Copying emails from one instnce of Thunderbird into another

My requirement is to copy emails from TB in USB to TB in PC and vice versa. But if we simply copy the mbox files from one instance of TB into another, will it not overwrite existing files? I wanted to append emails.

Suppose, if we check emails using TB of USB, and if we want to copy them to TB of PC, what do I do? Copying the whole Mail folder works fine, but it will overwrite whatever emails present in TB in PC, right?

I wanted something like importing Outlook emails. Create a separate folder called 'emails of USB' and then have its own inbox,sent,trash folders etc.

I copied the folder in Local Folders, changed its name to 'emails of USB' and pasted them in the Mail folder of the PC's TB. All those folders(inbox,sent etc.) are getting created. Even those files corresponding to each folder(mbox files for inbox etc.) is present with the same size. But then, no emails are visible in any of the folders. What do I do now?

Have I overlooked something? Is there anything I should do with the pref.js or user.js files?

Thanks in advance
Merrin