I read a report than an individual accepted both HPSP and FAP scholarships. After graduating medical school he received a full deferment for urology (5 years). He then applied for FAP during residency. The report stated the payback was consecutive. It is possible he ended up owing 9 years of payback: 4 years for HPSP and 5 years for Urology residency.

Note: Please take this with a grain of salt as it needs to be validated.