[skidlids]

Simon there have been a few different under seat exhausts done over the years
Kev Palmers had a true 2 into 2 underseat setup a few years ago, without any link between the two pipes, He made it up himself using the original headers and then cut up a set of my old R6 header pipes to make the rest.

Om 4 Stroke 4 cylinder,pipe length can prove crutial so exhasut pulses don't interfere with each other, get the lengths right and they can aid the scavenging of the other cylinders, get it wrong and they can suck out the fresh incoming charge during valve overlap and hinder power

On a 4-stroke 2 cylinder its not as critical and I can;t see any real harm in keeping the two pipes seperate.
With regard to length, think of it as volumetric length, a shorter pipe with a larger diameter and equate to a longer pipe with a smaller diameter. if your trying to match back pressures
These days you can map each cylinder seperately so slight differences in exhaust can often be compensated for