Advanced Forms does support sending to multiple email recipients when added to the Custom recipient field as a comma separated string:
Using the internal hooks and filters you can do anything you want. You just need to spend some time exploring the code to find the appropriate way to extend it.
If you’re using an ACF email field in some kind of workflow to define a forms config. I’d switch it out for a text field where you need comma separated strings because a text field will support any characters but an email field will be validated by ACF as a single email address.
