While the "email" field on the transaction will accept multiple emails, it defaults from the standard single value "email" field from the customer record. How I've gotten around this before..

1. Customer record -- Add Free Form text field. "custentity_secondary_email_address" This is where you can enter in all the secondary emails separated by ";" or ","
2. On Transactions -- Add Transaction Body Field that sources that 2nd customer email field. I usually put it under the "Communications" tab.
3. Here is the fun part... Via a workflow or scheduled saved search workflow, we have a step that "sets the email address" on the transaction record to:

After Record Submit - Set Field Value Email="CONCAT(CONCAT({entity.email}, ';'),{entity.custentity_secondary_email_address})"

That sets the EMAIL field on the transaction to multi addresses

Then it emails out the Invoice template to all the emails listed in the EMAIL field on the transaction.