In reply to Lukem6:
Its a very British thing, but very useful for lots of reasons. Done many pitches over 40m for instance?
Take Valkyrie at the Roaches (2nd pitch), quite hard to do on a single without lots of drag unless you're really confident. But a by product of that, if you have an unconfident second for instance, is that you could leave the right one free after the spike and then when you top out, they effectively get a top rope for the traverse.