Yeah a true X-pipe will join both pipes together on the inside to smoothen and scavange the exhaust gases just like a merge collector on a header
in an attempt to pick up a few Hp
An H pipe will scavange in the low-mid rpm range Vs. a Proper X-pipe merge will scavange in low,mid and high rpm range more efficiently than the H pipe..
That is why alot of the Nascar set-up's switched to X-pipe systems.
The other benefit is a reduction in exhaust sound decibels
Sterling Marlin won the 1995 Daytona 500 being the 1st use a X-pipe system in Nascar in order to give him the advantage of picking up a few more Hp than the other drivers.
the story about it is here.
http://insideracingtechnology.com/drgas.html
Most people don't even know you can the same advantages of an X-pipe just by using a Proper Y-pipe merge vs just cutting/splicing 2 pipes to together in order to join them to a single pipe.
You can pick up 2-7 hp just by using the proper Y merge depending on howmuch Hp your car makes and CFM you engine requires to where a single pipe might not support the Hp vs Dual pipes.