Shift the PSU(s) to the top, and you can fit 4 Mini-ITX motherboards in it - although then case flow might be an issue seeing as it only has room for one 80mm fan in the back
Is it just me, or does that actually have a removable motherboard tray? Why would you need one, really? (I've never seen a computer quite so big, my first computer was a Socket A 440MHz job, and it was normal sized)
Have a go at budget parallel computing, with multiple mobos instead of a dual socket board.