I have spent days collectively trying to work with async comms and no xtal. I can get lucky, but for a customer app. I would always revert to a crystal Why not use SPI for both. Use one pin to select your other PIC, and one to select your peripheral.Or, if the peripheral is looking for a low to select, set your outlying PIC as an enable high, and use the same pin. Just make sure both external units go into high impedance when deselected. I did this with a video on screen display. The PIC generating the overlay could only grab a little info between frames. I used shiftin and shiftout with very reliable results. Async can be unreliable even with a crystal in noisy environs, but makes it easy to hook up to a PC or GPS engine, etc...

Ron