I haven't yet investigated this very much, I thought I would post it here to see what others think.
An inventor recently posted his new engine design on the Crankshaft Coalition
I'm sure we have other guys who will know more about this type of design. Essentially, the camshaft and valves have been replaced with a second crankshaft and set of pistons.
There are plenty of diagrams on the site, plus information/history, etc. Here's the link: New4Stroke.com
I'm not sure about the performance/efficiency/feasibility, or the extent to which this has been designed in the past, but at very least, it's thought-provoking.