This is the best way I have found...
BEHRINGER: MX400
Use the Behringer and 1/4 to 1/8 phono plug adapters.
Its small and runs on 12VDC - perfect for mobile applications.
You will need an amplifier or an amplified speaker (Moto made one that is nice.)
If your scanner doesn't have REC or LINE out, you have to play with the level controls a lot.
If one of the radios is a two-way with hot audio out, you might want to buy a car stereo adapter that converts speaker level audio to line level with an adjustment point (hint when using these since they are stereo use one channel [L or R] for one radio.)