Hmm, I'm not an expert either (obviously), and it's been quite a few years since I've studied about the flow of gasses, so I might be talking complete b*llshit..
I'm not sure how much energy gets lost in the silencer itself, however most likely quite a lot of velocity does get lost there, due to turbulent flow, and well silencing..
With regards to velocity aspect.
The total pressure should remain largely the same between the two smaller tubes and the after single bigger tube, as total pressure is made out of static pressure and dynamic pressure (velocity). I'd reckon as long as the surface area comparing before (2 tubes) and after (1 tube) doesn't change by too much, there shouldn't be a big change. One could however say, if the diameter of the one bigger tube, was slightly smaller, than the two smaller tubes combined, that would increase the velocity of the gas and maybe even counteract the cooling down of it, whilst it vacates the exhaust. Obviously spit balling here.
On the other hand, if the surface area of the big tube is bigger than the two small tubes combined, the dynamic pressure would drop (velocity) and the static pressure would rise. Which is not perse what we want. That's enough thinking for me, for now, before my brain explodes