Connecting the two antennas together like this will not help - you end up with a multipath problem (signal will fade regularly). This is because you have two antennas in different places, one receiving at a slight delay from the other - connecting the two will add the signal. I will attempt to explain visually (this is crude, just to describe concept - imagine smoothly curved lines like in the second pic (red line)):
The signal between Antenna 1 and 2 will be out of phase, the result being something different to what it is meant to be. Imagine that effect transposed on to what FM signal looks like to the headunit (imagine it as voltage vs time):
The head unit is essentially reading the voltage change in the wire connected to the antenna (tiny variances in voltage induced by the magnetic field of the RF). It then demodulates this varying voltage within a selected frequency range (depending on what radio station you want to hear) to analogue audio to output to the speakers. The actual voltage in the antenna wire doesn't really matter, its more its ratio to the voltage of signal that you don't want (Noise). The important part is that the change in voltage over time, the red line in the pic above, is correct - which it will often not be if you connect two antennas together. So, by connecting two antennas you will have a higher signal "amount", but the noise signal will go up by the same percentage AND the signal itself will get distorted. Does that make sense?
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