As @tyblu says, it's all right there in the manual for the USI. In particular what you want is to use it in "Three-Wire Master Mode Operation." Take a look at Figure 61 on page 143 of the datasheet. The datasheet is even kind enough to provide an optimized (8 instruction) assembler routine (SPITransfer) for this exact purpose:
If you wire it up so that the you have a 2313 pin connected to the controller's "latch" wire (orange), the USCK pin connects to the controller's "pulse" wire (red), and the DI pin connects to the controller's "data" wire (yellow) - you should be good to go. Per the datasheet, the code example assumes that the DO and USCK pins are enabled as outputs in the DDRB Register.
Once the hardware is set up, all you need to do is generate a 12us high pulse on the "latch" pin, drop the pin low for 6us, then call the above assembly routine twice (saving the return value away) after each call.
I found this reference very informative regarding the (S)NES interface.