In reply to Somerville:
I was taught to first tie a long overhand- then tie in using a second rethreaded overhand.
This has the advantage that any load is not passed through the middleman.
In terms of distance apart, about 12m seems right as this makes it easier to keep the rope tight which means that if someone falls in a slot they should not go far! With this and many other aspects of mountaineering you have to make your own judgement- Bruce Goodlad's book is worth a look.