If both plugs have spark, and both cylinders have good compression, then your probably not getting fuel into the not running cylinder.
Try swapping the plug wires as the above poster said, it could be that you have a weak spark. Or else, you may have messed something up when cleaning the carbs, maybe they were put back together incorrectly.
Ohh, almost forgot, make sure the tube that connects the 2 carbs together is in place!! Always a big mistake. It should connect the left and rights carbs near the carb tops opposite the choke lever.