I'm not saying that it's "bad" but if your able to get double mongoose on your weps, then I see no reason why to not do it(aside from rogues and fury warriors).
Enhancement shamans are about Keeping ToT on their enemies for as long as possible to proc their flurry/WF/Mongoose. When Flurry Procs, WF procs faster, then mongoose procs Faster. It's a chain. Going double mongoose is a "chance" ability. But this isn't a rogue, so please don't go the MH Mongoose/OH agi way. WF+Flurry means faster attacks and twice the amount of the attacks. Which means an Enhancement shaman, above all other specs/classes, will have the best chance to proc mongoose faster than any other class.
Not to mention, Lava Lash is an "offhand" Attack. So your offhand wep is going to be hitting a lot more than all the other dual-wielding classes out there.
If you feel that it's better than go ahead, I'm just looking at it from a distance and seeing a good out come no matter how I look at it. Unless god doesn't love you, then you're mongoose Procs will happen left and right by going dualwield mongoose.