Burnins psu can power 2 boards easily... even 10 bbxx overclocked to 450mhz.
You terminated the boards correctly with 2 jumpers each on both ends of the stacking cable? Did you build the stackingcable correctly?
As i only have 2 boards together, they are both terminated like burnings FAQ.
And burning supplied the cable.
I bought everything to remove any chance of my fucking it up =)
Thats strange. Maybe try firmware upgrade in case an old one is running. Are both boards powered the same way? Maybe try changing the power cable from one to another just in case its the cable or the output from psu for that cable is broken. But when it works when you use it with 2 usb-cables instead stacked then i dont know whats the reason.