Well, the whole point of ReWire is to provide real time "virtual cables" to connect the two (or more) programs so you don't have to always do and export/import routine when you want to change something. The basic idea is to route all your tracks in Reason to the Hardware Interface at the top of the rack - these are the outputs other programs "see" via ReWire and thus are available for routing. Then you just create the according track structure in the Live and record away (or just playback through Live - no need to record the same audio into Live if you want to keep working on what's going on in Reason).