In reply to DobloDan:
You'll be needing a double pulley and make sure it is rated for steel cables (e.g Petzl Tandem Speed or Decathlon do an Edelrid one. Often can be rented locally at the site of the Zip wire. Last one we did in France we just rented the pulleys and used our own kit (I think it also included a short sling because the normal VF lanyards go behind it on a steel crab (also rented) to act as a brake if needed (gently pulling down on the lanyards).
When you do the Zip it's worth trying to see someone else do it first - some are really fast and need quite a lot of braking, others will leave you dangling mid air if you don't let it run at full speed to get across.