In the past, I had pseudo-pair programmed in order to test. We'd agree on the external-facing API and pair-program a slow in-memory implementation. Once that was completed, we would split off and one of us would write test cases against the API (fixing up the in-memory implementation as needed) while the other would work on a proper database-backed implementation.

I haven't done this in probably 7 years, though. Mostly because I haven't cultivated this kind of working relationship with my coworkers.