Yup. The sensor for my Cateye is about twice the size of the kind that mount on your fork. But it's one sensor that picks up both speed (from rear wheel magnet) and cadence (from crankarm magnet) and mounts on the left chainstay.

And it works.

My Vetta, also wireless for speed and cadence, has a fork-mounted speed sensor (to the speed magnet goes on the front wheel) and a chainstay-mounted cadence sensor, and is totally unreliable despite my obsessive fiddling to get the sensors as close to the magnets as possible.