Well it wo't always proc your mainhand.
It will proc on your next swing. So if you use shadowfang and BWTB there is a 11.1% chance of proccing a OH swing and a 88.9% chance to proc a MH swing.
When using BWTB with a CB there will be a 14.3% chance for a OH swing and a 85.7% chance to proc a MH swing.
The reason for the chance to proc a OH swing it the fact that BWTB swings faster than your mainhand. Meaning that over the course of the fight the time between the swings will vary (unless your joisting 100% of the time).
Lets see this example.
0.0sec: Your shadowfang swings
0.1sec: BWTB swings and procs.
2.5sec: BWTB swings and uses the proc
2.7sec: shadowfang swings again
In this case you got a OH swing extra.
But if we continue from there we get this (getting 2 procs in a row how lucky
).
2.7sec: shadowfang swings
4.9sec: BWTB swings and procs
5.4sec: shadowfang swings and uses the proc.
7.3sec: BWTB swings
Now you got a MH swing.
The chance of getting a OH swing in the same as the chance of getting 2 OH swings between 2 mainhand swings.
To calculate this we take the speed difference between your mainhand and offhand so 0.3 for shadowfang+BWTB or 0.4 for cruel barb+BWTB. And we devide that by the swing speed of your mainhand
so 0.3/2.7=0.1111 with shadowfang and 0.4/2.8=0.1429 with cruel barb.
The chance of a MH proc is just 1-[chance of OH proc].