Can you remove the rocker train with the head still bolts down?If not can you somehow get all the valves to close.If you can,inject air pressure into one of the cylinders with low compression and see if air comes out the plug hole of the other low cylinder.if it does you have a cross leak problem,if it doesn't then that is not your problem.While the air is still hooked into the cylinder listen for leakage out the tailpipe or out thru the intake manifold.Whichever one leak that valve has a sealing problem. John.