Is it possible to build 'Contra-rotating' EDF?
The primary reasons you'd want contra-rotating pairs with conventional (unducted) propellers are cancellation of torque/P-factor, and delivering more power with a limitation on diameter (tip speed, usually, or landing gear height).

Neither of these really applies to ducted fans; it's easy to null out the slipstream rotation with fixed vanes in the duct, and flow is always perfectly axial so there's no P-factor to worry about.  Further, pitch can be built high enough to absorb the needed power without supersonic blade tips, and generally is.

Bottom line, there's nothing to be gained from the extra weight and complexity of contrarotating fans in a duct compared to just using a slightly larger fan and motor to deliver the increased thrust you're chasing.