Just saw this example as well, haven't tried it though so no guarantees:

<a href="mailto:someone@yoursite.com?cc=someoneelse@theirsite.com, another@thatsite.com, me@mysite.com">Email Us</a>

This example implies that there can only be one primary mailto recipient and the place to specify multiple additional recipients, separated by commas, is in the 'cc' area.

Good Luck