The skew in sampling the 13 signals using a single ADC shouldn't be a problem, but if it is, you need either 13 ADCs which are all strobed at the same time, or 13 Sample/Holds, which are then multiplexed to a single ADC using an analog muliplexer. I haven't tried it, but should be able to start an ADC conversion (in the PIC), wait a fixed delay, and then read the ADC result register. The MSBs should get set early in the conversion process. At any given ADC clock rate, the minimum delay to get just the two MSBs should be constant...