Hi,
I think we're just talking above each other heads here.
Yes the MSSP module IS the module doing SPI so of course the MSSP-module uses the SPI pins on the PIC. But, again, if you're using SHIFTIN/SHIFTOUT to do your SPI communications then your are NOT using the MSSP module - even if you happen to have the MCP2515 connected to those pins.

If you're ditching the RF-module that frees up the USART to be used with RN-171 does it not?

The exeternal one you linked to has one advantage though and that's its internal 64 byte buffer giving you some flexibillity.

If you're still in a position where you can add external circuitry why not simply change the PIC to one with two USARTS (the 18F45J50 for example (there are plenty)) instead of messing with external ones? Or, again, use SERIN/SEROUT for one of the modules and the USART for the other.

/Henrik.