to be honest I have no idea why the headers on my SD990 join to become one and then separate again.
I suspect they do join them simply so you just need 1 cat and 1 O2 sensor.
I suppose with a twin set-up the silencers could be physically smaller and still have the desired effect noise wise, and be freer flowing..
I don't know I'd be interesting in forking out too many beer tokens though. I'd guess it'd be £400 at least, likely more?