Can two 12/2 cables be used to wire a 240 electric Fireplace?
Sure, if it's actually 240V and not 240/120 split-phase.  Difference is, the latter needs a neutral to power loads internal to the machine which need 120V.  The former does not. 

240/120 example: an electric oven with an oven light, where the light is a common off-the-shelf 120V incandescent.   To make this oven pure-240V would require the owner to find oddball 240V bulbs, or have an internal transformer to knock down 240 to 120 for the bulb.   

240V example: hot water heater or baseboard heater.  They simply do not need 120V for anything.  

All conductors must be in the same physical cable.

For a straight 240V service on a 20A circuit, 12/2 is fine.   There's one small gotcha: 12/2 is only made with black and white conductors.  Thus the white conductor needs to be painted or taped a band of color, to indicate that it is a "hot".  Previously, the law did not require this if the usage was "obvious", and as a result, many electricans fell into the habit of not ever marking anything.