If one of the systems has a tape record output you can use that to connect to the other system. If not then you can use one of these:
http://www.crutchfield.com/p_142LOC80/Scosche-LOC80.html
to convert the speaker out to line level out. You will need an unused input on the other stereo to connect it to.
If possible the better sounding system should be the one receiving the output of the other system.
You could also just add a Bluetooth receiver to the Aiwa and that would give you all your sources on one system.