The simplest way is to just parallel the two inputs up and hope for the best. It might work adequately but you might get some reflections that cause the FM stations to be partially weakened. It will barely affect AM reception in the same way because the wavelength of AM is hundreds of metres and reflections won't be an issue. However, the wavelength of the 88-108 MHz FM band is about 3 metres and a quarter wave is somewhat less than 1 metre meaning you might hit problems but it's worth a go in the first instant.
The next easiest thing to do is make a resistive splitter that takes in a 50 ohm input from the existing antenna wiring and splits it into two 50 ohm outlets, one for AM and one for FM. The only problem with this is that you lose 6dB of signal for each output but it might easily work.
The last simple option is to buy an RF relay that can be operated as a changeover switch thus you can route it to the AM input or the FM input.
A little more complex is a broadband low noise transistor amplifier that takes the antenna signal in, amplifies it by 6dB (or so) and feeds two outputs which connect to AM and FM inputs respectively.