I have Thermal for the 1G and the 4" tip is really 4" and it is too small. The 5" is probably perfect. Two ways to get dual tips are 1, to figure out a way to drill the muffler and weld on 2 tips, which would be a pain in the ass, if it is possible. Or to buy on of those tips that mates 2 tips that expand off a single tip, and then weld that on. I would go with the single 5". But when I do it I am going to scrap the entire thermal muffler and throw on a round ultra flow with one of those 5 or 6" super short stack tips.
-JOHN