The accessory jack is only for transmitting. Like plugging in an iPod or some other digatla player. I don't know of a way to wire through an FM transmitter raido (like a motorola) to use the ultra headset to talk through.
If you are going to the expense of getting an FM talker, the additional headset is not going to be that much more. You would probably have to turn off the intercom /CB on the ultra but you wind up with a headset on each side.
Might just be easyier to get the second bike a hands free walkie/ talkie & use the cb on the ultra - that way they have their own head set.
Let me make sure I understand. That means the Ultra rider would have 2 headsets on?