Noiz2
Sound Ninja
You can use two of these at a time and they don't conflict. You just have to make sure you power on one matching set at a time, have them pair, then power on and pair the other set.
You could probably use more than that, but on some shoots (especially ad's for some reason) there will be expectations of a lot of units being handed out. Cost wise these would be great but having 6 or so transmitters stacked up around your rig would be kind of klunky.
On a big job you probably wouldn't be handing these out anyway, but those local ad's with visions of grandeur where everybody wanting a headset hanging around their neck...
But handy to have anyway. I will probably pick a couple up just incase.