May one eat in a Sukkah which has the Sechach of an upper Sukkah protruding over it?
If the bottom Sechach is firm enough to hold a person lying on pillows and blankets: This matter is disputed by Poskim. Some Poskim hold that it does not invalidate the Sechach directly under it being that this protruding Sechach has no walls. Others however rule that despite the above it does invalidate the Sechach under it. Practically one is to be stringent and avoid this situation.
If the bottom Sechach cannot hold a person lying on a pillow: The lower Sukkah is completely valid.