So unless you're training or a "Strava nut" does it really matter?
If it does then does moving the sensor from bike to bike more trouble than buying 2 more sensors?
I have 2 bikes and the Garmin supports multiple bike profiles. On my road bike i have a single sensor that does both cadence and speed. On my CX bike I only have a cadence sensor and speed is via GPS. I never noticed any inconsistent speed issues.