SPI Interfacing

I am working with 2 Arduino Due boards and trying to make them communicate using SPI. I am not able to make the Due bard work as a slave. I'm wondering if anyone has made any progress on that matter. I have tried the code is the following link but have had no luck in making it work. https://github.com/manitou48/DUEZoo/blob/master/spislave.ino

If anyone has a working code, please help me out.

Yes, there's a lot more tutorials on I2C slaves. It should work just fine with the UNO code posted by Nick Gammon here: http://www.gammon.com.au/forum/?id=10892

I haven't tried it.

I am actually looking for SPI and not I2C. I want duplex communication with multiple slaves.

Yes, that is what's in the link.