Most flip-flop hubs are only threaded for a lockring on one side, but the sprocket/freewheel thread is the same, so you can screw a fixed sprocket onto the freewheel side. I'd put the smaller sprocket on the side without the lockring, because it's less likely to come unscrewed
http://sheldonbrown.com/fixed.html#fixedmtb
Most flip-flop hubs are only threaded for a lockring on one side, but the sprocket/freewheel thread is the same, so you can screw a fixed sprocket onto the freewheel side. I'd put the smaller sprocket on the side without the lockring, because it's less likely to come unscrewed