First of, PCM1704 has been discontinued for too long for FiiO to source enough quantity. Even HifiMan has trouble sourcing enough last year to keep the HM801 production going, so they have to stop for a few months. It will be very unwise for FiiO to use PCM1704.
Second, there is no such a thing as single vs. dual PCM1704. It is a single channel chip, so you always need two chips to have a stereo signal. PCM1972 that is going into the X5 is already a dual channels chip, and I'll rather have one optimized chips rather than randomly stuffing two chips together just for marketing purpose.