ok. this might seem like a weird question but is it possible to make your curve split up and join itself again. for instance lets say i was extruding a square along a path but i wanted a fork to then open up in the path and then join itself again. is this possible?
this is the tutorial ive been using: http://www.youtube.com/watch?v=FUNZobxP4Sg
No, as far as i know. But you could make another curve segment and connect manually when converted to the mesh.
ill give it a go 
im having trouble joining the two meshes. ive got the meshes as different objects and ive been trying to join them with the boolean modifier with the union setting. but what happens is all the curves keep disappearing. is there a reason this might be happening?
also. ive just done a quick test render in cycles of the objects ive made with the curves and they dont show up in cycles 
In object mode, select two objects (of the same type) and ctrl+J to join them.
No way of knowing why your curves won’t show up in Cycles, works here
its ok, i fixed it. i had to change the object setting from ‘frames’ to ‘none’
thanks though