Neither the HTML "mailto" nor the emailfilter-droplet is created (designed) to send an email to 2 recipients.
To distribute one email to two mailboxes (where ever they are) you should use a server-side solution eg the virtmaps table in sendmail or postfix:
info@yourdomain user1, user2
This means all mails to info@ distirbute to user1 and user2.
Which means local users on this server.
But you can also do something like this:
info@yourdomain user@domain1.tld, user@domain2.tld
Here are the users not local on the server.
Even you can mix it up.
IMHO this is the only honorrable way for distributing email to more than one user.
At last let me cite from nice site:
HTML mailto screams "this web site has been developed by a beginner"... A professional approach would be to use forms instead, and these require knowledge of server-side scripting.
Regards Bernd