QT/tandem breaker wire gauge
With two adjacent regular breakers, you can share the neutral, a multiwire branch circuit.  Adjacent breakers are on different legs, so the neutral only carries the difference in current between the two hots due to a cancellation effect.  

With a tandem breaker, both breakers are on the same leg / line, there is no cancellation effect on the neutral, so you can't share the neutral - you'd be doubling the load on the neutral conductor!  

So yes you must use two 14/2 cables rather than a single 14/3.