I have seen video of a guy who carries a single mtn bike on a special mount on a BMW GS1100 (big bike). It was off to the rear on the right side and was pretty amazing to see... but I think once you get two out there it would be too much weight and windage in totally the wrong place. I do not see a viable way to carry two unless you go to a motorcycle trailer.
Well Googled the link - this is the bike:
http://www.timberwoof.com/motorcycle...keCarrier.html