Depending on the physical size of the two units being installed, you may not have to enlarge the opening at all. I know Spaceman Spiff has two DIN units, and I'm fairly sure he didn't enlarge the opening.
A mounting kit can be had from <a href="http://www.scosche.com/scosche_caraudio.aspx?CategoryID=5&ItemID=FD1330B" target="top">Scosche</a>. I know there are places to order online, I just can't seem to find any at the moment...