For all of you that need two SPI ports, two SPI ports are already embedded in the "regular" ATMega328p. That is if you don't need the USART. The USART can be configured to operate in MSPIM (Master SPI Mode). By the name of it you can conclude that the device only operates as master. But I guess that is fine for 99% of all MySensors applications?

Here is an Atmel application note concerning this matter:
http://www.atmel.com/Images/doc2577.pdf