Measure speed and cadence with Duotrap sensor and Garmin 310xt

I am assuming the sensor you got is the combo speed/cadence sensor, right?  I'm surprised that it didn't come with the appropriate magnets...that seems weird to me.   But to answer your question, yes, you would need both a wheel magnet and a cadence magnet.  The wheel magnet, obviously goes on a spoke in your rear wheel...and the cadence magnet attaches to the right-side crankarm.  I have a friend who rides a Madone 6.2 with the built-in duotrap sensor and his cadence magnet is attached with a small, thick rubber band...however, I do believe he had to remove his pedal to put it on.