You should not do that. The integrated hybrid drives have microcode to ensure that there is no dataloss. Using 1 drive as a buffer for another much larger disk would introduce all sorts of buffering issues similar to cache hit/miss. And given that they are descrete devices where all the communication must take place across the same sata bus, data loss is highly likely (hangs, etc would potentially have part of the data on 1 drive, part on another, etc)(