Do the RAMs have identical specs (timing, speed, voltage, model number)? Dual channel (sold in pairs) would be better.
Use the 2 dual channel RAM in pair, then put in dual channel configuration in the slots (usually the same colored slot). If that's still won't work, try only 1 stick at a time in different slots. If only the first is usable, like you first said, then there's no helping it. You're limited to just one slot then