I got a 513 about three months ago and found it perfect for my needs.
BUT there were times when two programs that I wanted to record were on at the same time. So I bought a 515!
The remotes have a very wide beam. Even with several feet of separation between units they both responded to the remote.
After reading the posts suggesting covering the receptors on the units (tried it), here is the solution to controlling one unit at a time.
1. Get a standard ball point pen, the kind they hand out with advertising. 2. Choose a pen with the longest end that holds the point in place, preferably black.
3. Take it apart and pull the pointed end out off the case.
4. Adjust it to fit over the LED in the remote. If it is too small, dip the big end in hot water and enlarge the opening.
5. After mounting it on the remote, you can adjust the distance between the units.
I used masking tape to hold the "Beam Spread Reducer" in place till after the warranty expires, then it will be permanently mounted to the 515 remote, which also controls the 513.
I have four feet horizontally between the 513 and the 515. That gives about a foot of dead zone in the middle, so they could be closer together.