Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Welcome to JUST ANSWER small engines, let me try to help you !! I don't know it all, but maybe together we can get you a satisfactory answer!
With a Briggs twin, having spark and compression, I would check the valves on the dead cylinder, and I expect you will find a bent or broken pushrod.
I see question is still open, did you find problem?
Model number? You can still check the valve clearance through the crankcase vent covers on the front of the engine.