You can send a test email to multiple users from the admin interface if you use “mailaddy1; mailaddy2” in the to: field. Both mails arrive.
BUT
if you look at “Sent” it only shows the first mailaddy.
Noticed this while chasing down a bounce/blacklist problem.
Sorry to contradict you, but the former (/admin/email) does in fact work with two mail addresses separated by "; ". The same test arrives at both email destinations, but the list in “/admin/email/sent” only shows the first of the two emails.
The multiple test email is great, by the way, it helped me deal with a user who was having problems with his ISP blacklisting us, so please don’t fix that part!