GPS parity decoding, how to get D29 and D30 of previous bits?
You need to take in the first word of the subframe and ignore every thing in it except D29 and D30 which you carry forward to the next word. 

It will probably be necessary to comprehend the higher level framing protocol that defines frames and subframes. No doubt at some part of the protocol framing boundary the previous D29 and D30 are assumed to be '0' or '1' for the first word of the frame.