Multiple antennas vs signle antenna
An antenna's impedance is matched to the transmission circuit in order to produce the best output level (within regulatory rules). Putting two antenna's in parallel at best shares the power. At worst, they together get half the power and even worse they may produce fields that cancel each other's transmission and you get very little transmission distance.

You might get lucky with some antennas and get an improvement but that improvement will be more directional than with a single antenna. This may prove problematic in itself because you have to be facing the quadcopter more precisely than before (and it may still break regulatory rules).