Hi there , please post back model and type codes from label on flywheel cover ie something in this format
Ech740-3132 then sn ###########
It helps when diagnosing problem
But in the mean time,as you state, both cylinders have spark , but is it at the correct time ? do both cyls have fuel gettin to them ? Do you know how much compression each cylinder has?, ( by the way a leak down test can locate and confirm a blown head gasket or leaking inlet manifold gasket ) a leak at only one end of intake manifold will only affect the cylinder closest to it , a blown head gasket lowers compression and draws in air on intake stroke so cylinder with blown head gasket may have some compression and good spark , will not when cutting load placed on it , power on that cylinder drops and engine runs 1 cylinder , in case you were wondering a engine will run ok on 1 cyl as long as load does exceed power output from remaining cyl ,if it does engine stops , pls post model and trim code numbers and will help further
Cheers