Take a timing light to the coil, when it br,br,br gheck to see if it double firing a cylinder or firing late. If so its either a pulser of CDI...seen this problem on Merc 4 cylinders and bad bias reference from trigger
Either the lower unit took a crap or bearing failed on upper crank. This will dump the rollers in on top off crank offset and prevent rotation. Go simple and check lower oil for metal.
Your tachs are off...you have 2 cylinders connected to crankshaft. If top cylinder reads 5000 RPM the bottom cylinder is moving at 5000 RPM, there is no way a cylinder can have lower RPM than the other.