I have an older Shebeest skort that I commute in sometimes (and race, rarely). But it's very short. Too short to really get stuck on the saddle. And the shorts that are attached are shorter than any of my normal bike shorts.

It's useful for commuting because men seem more inclined to stop and give me the right of way (when due) than just to fly through every crosswalk, red light, etc. That being said, I also get pretty self-concious wearing it. Even though it hides more than ordinary bike shorts, it's essentially a short, tight, black, miniskirt. The tights/shorts combo you're considering probably wouldn't feel quite as immodest!