I did this with David's once: after I found out they didn't immediately have a member with the points to satisfy my reservation request (and that the resort/room category I wanted had dwindling inventory), I asked them to waitlist me, and ALSO informed them I was making the same reservation request with DVC Rental Store, and would take the first reservation I could get with either company. Then I watched my e-mail like a hawk. A few weeks later, David's sent me a "congratulations, we got you your reservation" e-mail. I immediately called (as in, within 60 seconds of seeing the e-mail, I was dialing the number for DVC Rental Store, which I kept next to the phone) and canceled my request with DVC Rental Store.
As
@nickys warned, had I not been checking my e-mail frequently or been unable to act immediately when I received word they'd secured my reservation, there was a possibility that I might have ended up with two reservations -- one through each broker, at which point I COULD have ended up "on the hook" for both, since generally, payment is due, in full, immediately upon the securing of a reservation. If you're thinking of doing this, I'd suggest calling both brokers ahead of time to find out exactly what their policies are if you somehow ended up getting simultaneous reservations with both. (*Note: I've called David's and DVC Rental Store multiple times in the past. Their employees are friendly, knowledgeable and eager to answer your questions, so don't hesitate to contact them if you still have concerns after checking out their websites! You'll get ahold of a real, live, English-speaking, on-site employee -- not someone in a call center thousands of miles away.) Given that a DVC reservation is usually a non-refundable, immediate-payment-required deal, having 2 requests at once may not ultimately be worth the risk.