In the 80s where aftermarket car stereo was really dabbling in HIGH END, some of the FM receivers were so sensitive that they had HUGE problems with multipath. As an installer,  I would often take out the crappy old factory stereo and install a state of the art Alpine, Concord, Soundstream, Proton or other good stereo and the customer would return with reception problems that they did not have with the old factory stereo.. FLUTTER...    Caused by the FM signals being blocked and bounced by buildings in the city.

Trying to get rid of this problem without desensitizing the tuner or cutting off high frequencies (like the factory stereos) they came up with the diversity idea.
The diversity radio had the ability to sample both antenna inputs and automatically switch to the antenna that had the best signal..  

A few factory car stereos such as some Nissans also made some diversity tuners, these cars have two antennas ..

At this time I am not aware of any aftermarket car stereo that uses two antennas..  So the customers question was, How do I hook in the two antennas to a radio with only one antenna port?

Now they do make an adapter for the dual antennas, but my solution when I was installing and before they made the current adapters was to simply use ONE of the antennas from the car.   The aftermarket stereo has only one input so it cannot possibly SAMPLE two antennas and choose the best one like the original diversity radios were said to do. So why would you even need two antennas in a normal stereo?   I don't think you do.  Just pick one and give it a try, or pick the other one and see if you like it better, or buy the adapter available now and try both at once (not truly diversity, just two antennas) and see if you think the receptions is better or worse.