Looks like I'm wrong, I thought an X pipe didnt join but apparently they do
Copied and pasted from elsewhere:
With an H-Pipe high and low pressure pulses from the left bank will combine and become friendly with the other bank. The low pressure pulses on one bank draw high-pressure pulses from the other, called quasi-static scavenging. Makes more torque (than no crossover pipe at all), and smooths things out acoustically.
The X-Pipe though, does exactly what the H-Pipe does but better, different. Instead of letting pressure pulses go back and forth across the H the x-pipe makes sure the gasses from each bank ALL get very intimate together causing dynamic scavenging.. Thats why it sounds so different. Its smoother too, since the energy from bank-to-bank pressure fluctuations are used more effectively to scavenge the gasses. Less wasted energy means less sound, but more power.
Readers digest:
For deeper exhaust tone, sound -- H Pipe
For performance -- X Pipe