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 =)