Here are the floor plan and pictures of the corner sink(s) in my garage darkroom built last year. The long "process" sink (37 1/2" wide) spans a 20 foot wall to the corner where it drains by tilit in both directions towards the drain pipe in the far corner. The smaller "wash" sink abutts the long sink and its drain feeds into a common drain coming from the long sink and into the house plumbing on the other side of the short sink wall (I was "lucky" there - you're main problem would seem to be the pump necessary to lift water to your existing drains. The contractor (...don't get me started) built the 2 sinks out of 3/4 inch marine grade plywood, glued and screwed. I waterproofed the sinks with several coats of (white) marine gel coat. The gel coat is difficult to get right so I would recommend other options (marine epoxy?)