There are some others but these are the first that come to mind and probably the most popular. They tour.

http://www.theeroticreview.com/discussion_boards/viewmsg.asp?MessageID=856598&boardID=12&page=#856598  Scan the rest of the thread.

I know fraternal (non-identical) sisters who tour independently, quite possibly to avoid being asked to work together. They both do GG but not with each other. Some of those GG reviews are off the charts.  

In some cultures (I'm thinking Asian), a good friend is sometimes referred to as "sister." I've visited hostesses who introduced me to a roommate or travel companion as "This is my sister, Name." They sometimes work together but are not genetic sisters.

I'm thinking of another pair of real twins but I'm blanking ...