By default, Send Separate Emails is selected. However as soon as someone selects a second recipient it is greyed out and cannot be deselected. Makes sense when the template has merge fields but if the user removes the merge fields, the box is still greyed out.
It isn’t clear to the end user anywhere that they need to save the draft and exit the editor, and then it’s available to deselect when they return to validate. Maybe it isn’t obvious because this isn’t how it should function - instead, as soon as the merge fields are removed the box should become available without requiring the user to exit and reopen.