OP, you could theoretically run into the situation of both agencies having Owners make reservations for you.
From the point of view of an owner who has rented out points through David's:
Step 1: I notified David's that I had points to rent.
Step 2: Twenty minutes after finishing the process of registering with David's, David's emailed me this:
"Congratulations! I currently have a guest interested in a rental and I have confirmed availability for their request. I am sending this request out to a few members in order to obtain the quickest reply. Are you available to make a reservation in the guest's name if I send you the details?"
The email included how many of my points would be used, and the check-in date.
Step 3: I emailed David's that I was willing to make the reservation.
Step 4: Ten minutes after that, David's emailed me with the guest's information so that I could go online to the DVC Member website and make the reservation.
I don't know if, in between these steps, David's contacted the person who wanted to rent points, and asked them if they were still interested in renting. It didn't feel like David's did that, because very little time elapsed between the steps.
I don't know how
DVC Rental Store works. I think it might be different. If it works the same way, then theoretically, agency # 1 could make a reservation for you. Let's say something was going on in your life that day which prevented you from checking your email, finding out that the reservation had been made, and pulling yourself off the waitlist for agency # 2. (You're sick. Somebody in your family is sick and you're tending to them, or you're in the ER with them. You're stuck in a lengthy meeting.) Then agency # 2 has an owner make a reservation for you. Now you have 2 reservations. Maybe this highly unlikely, but you need to think it through in light of the cancellation policies at both agencies (which you need to review).