I did a "no cost" tail tidy which retained the original lights/number plate mount and involved minimal (and reversible) damage to the rear of the bike...
http://www.xt660.com/showthread.php?t=18310
Scorpion do a twin can system that looks quite sweet and follows the exact lines of the original pipework...
However, I'm not so sure the pipe linking the two sides will clear the tyre on full compression though as its much further to the back than on other twin pipe systems??