“There are no safe means of assigning multiple recipients to a single
mailto: link via HTML. There are safe, non-HTML, ways of assigning
multiple recipients from a mailto: link.”

http://www.sightspecific.com/~mosh/www_faq/multrec.html

For a quick fix to your problem, change your ; to a comma , and eliminate the spaces between email addresses

<a href="https://stackoverflow.com/questions/13765286/mailto:person1@domain.example,person2@domain.example">Email Us</a>