If I have multiple backups for multiple email accounts, I can tell from the file name which email account it belongs to and import the correct one.
They are all called delta-chat-$date-$counter.tar, so I have to guess in which order I exported them to find out which backup I want to import.
in 1.42, the backup names will include the email address
in the form
moreover, the 1.42 android version will have an option to export all accounts together