You can order the Watkins at harp.com. As for the duo, there are many pieces arranged for harp and keyboard. (some might be piano or organ, but usually they can be made to work, as can duo harp arrangements)
The easiest way to find the method is to do a search on their site for Watkins, then click the methods and studies link at the top of the results. It pops right up.
I would suggest any of the baroque or classical-era concerti for harp and harpsichord. Also the Blano or Blanco Concerti for 2 Instruments, published by Schott.