That device doesn't give you a continuous 3.3v. It uses the accumulated RF energy to charge a capacitor, then when it hits a set voltage it boosts it up to the voltage you want until the cap decays back down, and then it shuts off the output. Since the output is intermittent, you can't stack two of them and expect to get 6.6v. Most of the time you'd get 0, sometimes you'd get 3.3 from one device or the other, and very rarely (I'm assuming, I don't know your load or what kind of duty cycle you'd get from this device) you might get 6.6v.
The two devices obviously couldn't share a ground, you'd need to stack one of their grounds on top of the other's Vout, and then take care of the myrid of problems associated with doing so (your two RF antennas wouldn't share a ground, your digital signals wouldn't share a ground, etc).