Two antennas in parallel
These are two antennas in parallel. On the low frequency, the longer one is a quarter wave and the shorter one has a high reactance which shunts the drive port. The longer one is adjusted in length to have the opposite reactance. On the higher frequency, the short one is a quarter wave and the longer one is a half wave which makes its input impedance nearly infinite which has little effect on the drive port.
You may have seen scanner antennas with three parallel elements if different lengths. they use the same concept.