Hi Sheldon,
You can't do that with SHIFTIN/SHIFTOUT (without modifying the underlaying ASM routines of course (don't do that)).
You need to write the routines yourself, shouldn't be that hard to do I don't think. But, as Richard points out, using the MSSP module in SPI mode will probably give you equal (or better) performance compared to a bitbanged dual SPI setup.

/Henrik.