Your test of putting oil in the two suspect cylinders isn't really really going to tell you much, as there's only gravity forcing oil past the rings, but while running, there's both compression pressure and the movement of the rings in their grooves that will help oil get past. The oil control rings are supposed to remove most of the oil splashed against cylinder walls back down to the crankcase, and if they aren't doing their job, that oil can get past the compression rings into the combustion chamber.
Typically oil leaking past valve seals, especially on intake valves will cause smoke on first startup and on deceleration when vacuum pulls the oil past the oil seals. I don't recall if you posted about smoke out the tailpipe.
But cleaning up one of your heads and maybe renewing the valve seals on it, along with lapping the valves like Kevinski suggested to check the seats, would probably take the possibility of the issue being valve seals out of the picture.
If you're torn down far enough, honing the suspect cylinders and replacing the rings in those two might be another step in the right direction. But oil is still relatively cheap compared to a complete overhaul.
