I have light switches inside my house (2) that control 2 sets of front porch lights. Both switches are 2 way single pole. Two black wires, one ground for each. I've figured out how they will attach to the timer, but it leaves one question. One black on each switch combines with the other then combines with the same wire on the timer. Since there is now a ground on each switch, which attaches to each switch (plastic box), can I attach both grounds to the screw on the timer or do I just attach the two grounds to each other and cap with a wire connector?
I have not had good luck trying to combine grounds by the use of a switch grounding screw. I also suspect this to be a code violation. I prefer criming all ground wires together with those copper bands, or use wire nuts.
Based on your question, however, it sounds as if you don't have a copper ground from your supply. Is this true? Grounding devices to each other but to nothing else would not serve much purpose. Make sure all your switches and timers are grounded to the copper in your supply wire (assuming you have one).