You are creating a New E-Mail activity by using Workflow rule and it is able to send E-Mail correctly. Now add an extra recipient, when you check the E-Mail activity the recipient is added succesfully.
However the E-Mail is only sent to one recipient only, althought there are two in the activity.
Workflow Email Activity creation doesn't rely on the recipients of the Email Activity. It only relies on the Workflow recipient lists.
So if additional recipients are added to the Email Activity, this would not result in sending emails to multiple recipients.
This is expected system behavior
KBA , SRD-CC , Cross Components , How To