Acymailing v5 is completely being recoded and that's version 7. Look at them as two different extensions, each using their own set of database tables. That's why you can use both on the same site and you actually have to migrate your data from v5 to v7. All database tables starting with #acymailing belong to v5 and the ones starting with #acym belong to v7.
Please not you would need to recreate your subscription forms with the v7 ones. Otherwise new users keep being signed up to your v5.
Once you're satisfied and want to use v7 for production go to Joomla->Extensions->Manage->Manage, search for Acymailing and uninstall everything that has a version number lower then 7.