Are the wall switches on opposite walls, or are they both in site from the main seating position? If they are both anywhere near close enough to see without straining your neck, the Lutron Spacer system is relatively cheap and couldn't be easier to install and program. The IR from the Pronto is very powerful and can hit all switches with one button press "most" of the time. You could use an IR repeater/extender of some kind wired to each if they are not easily in view of the remote's IR signal. If they are in site, it is very reliable. You might discuss this option with your client if cost is a real factor. Otherwise the RA system is exceptional.

One note on the Spacer: I was able to operate two lamp dimmers (external spacer devices) that were located on opposite walls almost without fault with a Marantz RC5000i. The walls and ceiling were white, so the IR bounced around well enough that it really did not matter where I pointed the remote.