A lot of people once read some math that they never understood, and then just repeated it.
Having WF on both weapons DOES NOT increase the individual swing proc rate. What it does is make the odds that AT LEAST ONE of your two swings will proc WF, 36%. (This is a very simple probability calculation - odds of no proc on two swings is 0.8 * 0.8, or 0.64. If the odds of not occurring is 64%, then the odds of occurring is 36%.)
WF on both hands is simply because even an offhand WF proc is better than any of our other weapon imbue options. Even with the cooldown preventing your MH from proccing on its next swing; it’s only 20% odds, it probably wasn’t going to anyway.
As for speed matching, that’s because of Flurry. It does not give you two swings per charge. It gives you a fourth swing per proc, instead of the normal three. Additionally, having speed matched does make it marginally more likely that a WF proc will be MH, because when your weapons hit at the same time, MH damage is always calculated first.