Two stops is not a little mistake, and two light meters erring by two stops is diabolic a coincidence.

Maybe this is a daring question, but are you sure you use correctly the light meters?

For incident light metering you normally have to put the translucent dome in front of the cell, walk to the subject, and point the light meter toward the camera.

For reflected light metering you normally have to set a translucent dome aside (or take it out etc.) and you meter from the camera toward the subject.

When comparing two incident light meters have both instruments either in shade or directly facing the sun. The weighting of "peripheral" light might be different and when the sun is 45 or so the two instruments can give a different reading.

Maybe this is all already very clear to you.

Fabrizio