Atmel’s application note (PDF!) Using the Master SPI Mode of the USART module:
• Enables Two SPI buses in one device
• Hardware buffered SPI communication
• Polled communication example
• Interrupt-controlled communication example
For the majority of applications, one Serial Peripheral Interface (SPI) module is enough. However, some applications might need more than one SPI module. This can be achieved using the Master SPI Mode of the devices with this feature such as Atmega48.