I own a raised ranch with two kitchens on top of each other. When we run the dishwasher upstairs and it drains the sink downstairs fills up to overflowing. This only happens periodically but recently quite often. I cleaned the trap and it continues to happen. Next I used a liquid commercial drain cleaner ( similar to drano ) and it continues still. Any advice??

You have a blockage or partial blockage in the drain line serving both kitchens. Snake drain line from the lower sink.

OK so I snaked the drain, went in about 20 feet could not get anymore in. put trap back together. Ran water, backs up and it does drain, just very very slowly. Before it would not drain.

So now what?