Attempting to connect two #14 ground wires to a single ground screw in single-tube Lithonia fluoresecent fixtures joined end-to-end resulted in stripping the screw threads in the sheet steel fixture. Is it acceptable to substitute the next larger size of self-tapping machine screw. Of course it won't be green and will be cadmium or zinc plated.
Alternatively, a longer ground might be screwed through a nut on the backside of the fixture but this would be inconvenient.
David Merrill