Too much air flow equals not enough back pressure which equals loss in low end power, so be careful how you do this mod. Also I know on the new Honda CR250F (dirtbike) they have gone to dual mufflers for sounds reasons..it is quieter than before.
Looking at car technology, 1 single 3" pipe was equivalent to dual 2.25" pipes and made no real difference in power. There are charts out there for determing what size pipes dual v/s single you need to support a certain amount of HP.
IMO it is more for looks on the SV650 than power unless you have a lot of extra goodies on the bike like a big bore kit, different cams and so on. Maybe someone else with experience on these bikes will know. If you do it (hich I think a few have) I would be curious to see a dyno report before and after the mod using the same pipe that was used on the single setup but have two of them on the dual setup.
Duals is primarily for looks/sound. Sometime they're used because there isn't enough room for a larger single exhaust.
Single is ALMOST always better.
As I said in the other thread.... cost, weight, complexity and flow all work against duals.
Stay single but marry if you need to.
The jap import Kenz Sport dual I had on mine for a while was just outrageously loud... To the point that after I did the cam swap, I just couldn't live with it- I was going to get killed by my neighbours
The thing about the duals for the SV is that as far as I'm aware not one was really designed with peak power in mind. If perfomance were your goal you'd not be close to doubling the weight of your exhaust system after all. The Renegades are essentially a very cool, expensive slipon (with a kludged mod to the exhaust collector too) while the Kenz Sport was totally built for torque and midrange (which it did very well) Most of the others have quite compromised pipework.