Commerce: notification emails to multiple custom recipients fails
I've run into the same issue and reported it to Craft Commerce support at the beginning of April. It was acknowledged to be a bug: 


  "At the moment we are only able to support a single email address in
  the custom 'TO' field."


and improving it was added to their TODO list.

You can use BCC field (with email addresses separated by comma or semicolon) to send notification email to multiple recipients or you can create multiple emails (hooked up to the same template and status) - one for each custom email address.