Yes, there is indeed a known loophole in the Flurry mechanic, where with 2 identical speed weapons, you end up hasting 4 white hits.
I'm pretty sure using 2 weapons of different speed will de-syncronise the white hits, and you will end up only hasting 3 white hits, as it is the intended effect of Flurry. Testing this can be a bit ambiguous, but you should be able to verify it by going on the dummies with 2 weapons of different speed, and checking how fast the Flurry charges are getting consumed (on the 3rd or 4th white hit).
Now, whether this is "bad" is an entirely other matter:
With the current amount of melee crit most enhancement shamans are sitting at in raids, the Flurry uptime should be close to 100% wether you have weapons of the same speed or not. And having a 2.7 mainhand over a 2.6 (assuming same DPS) will outweight by far the small loss of Flurry-hasted hits.
I did very simple test via EnhSim to verify this:
Sim 1: 2.6/2.6 weapons, 41.50% melee crit (my usual config)=> 95.34 % Flurry Uptime.
Sim 2: Took the same config and simply changed mh speed to 2.7=> 94.16 % Flurry Uptime.
So indeed less uptime in Sim 2 as expected, which confirms that Sim 1 was getting extra Flurry charges. But the difference was too minor to affect overall dps, and the benefit of having a slower MH resulted in 47 more DPS in Sim 2.
Hope that helped.