hi,

UART has a shift-register mode...

Indeed, Andy. What original poster said is:

Say I have two microcontrollers ..and I just want to have SPI communication between these microcontrollers for data transfer.

Okay, assume he does not need with SPI (it seems from his another post). Anyway I doubt it is possible to use shift register mode for data transfer between two MCU. The reason is that in this mode for data transmit MCU does both shifting data out and provide clock. And for data receive MCU shifting data in and provide clock as well. As result, neither transmitter nor receiver accept clock and no sync happens.
Well, I can imagine a trick which may help here but it is out of the subject here.

By the way, it does not mean that UART mode 0 cannot be used for SPI communication. As I once said, it may be used to connect to slave devices in SPI mode 3.

...as well as the "conventional" asynchronous modes.

Sure! And here it is the best solution for standard derivatives.

Regards,
Oleg