Jet's don't need counter-rotating as it's pretty much just direct thrust. Also, even if you could "feather" the front blades of a turbo-jet, the whole frontal area is still drag anyways. Even the High-Bypass ratio engines I wouldn't think it's going to make a huge difference if you could stop the rotation of the blades.

There are counter-rotating configurations with some TPE-331 installations. I think the Cheyenne 400LS did, but can't remember. I know I've seen it on something. I can't think of any PT6 equipped bird that does this though.