The PTT switch is for the CB. The intercom is voice activated. If your co rider wants to talk on the CB than they need a button also. They are plug and play installation, very easy to do.
But you can push your button and the co rider can talk on the CB. .On the radio there are several knobs, one is for INTERCOM VOLUME and one is MUTE. The Volume of course sets the volume of the intercom and the MUTE sets how easy the radio will mute when someone talks into the intercom.
Everything Joe told you is spot on. The only thing I want to add is unlike Joe I ride 2 up 99% of the time. If you spring for the seperate passenger controller 2 things come as a benefit. #1 Wife/passenger can control the volume of the sound system (they can only adjust lower they cannot raise volume) which helps if you like to rock & little lady is more mellow. #2 Although as Joe said you can key CB PTT I can tell you it becomes a bit of a pain in the %^* when she says "push the CB I want to say something" & then you think she's done & she starts talking again. I think you get the picture. Any way J&M makes a great addon. The following link shows it. Spring for the J&M. 3 years ago I bought the cheaper one & I just mounted the J&M this past weekend the quality is usurpassed & it also has a Mic shutoff switch if you're getting wind noise from passengers mic. Sorry top ramble just wanted to give you one ole Fossil's opinion.
I looked at all the options and read recommendations. It seems that J&M and Edsets are the top contenders for headsets. They start at about $170 and go up. I didn't want to sink that much money so I looked on eBay and found some for $79 each. I received them and installed on the helmets (we have full face helmets). We just rode last evening for the first time with them and they worked well.
All you should need are headsets compatable with the intercom system for your 06 pin connections. I have only owned one set of headsets, J&M. I originally bought them back in the mid 90's for my 1200, they've been moved to new helmets over the years and I'm still using them today. Agree with Fossil on the passenger controller, I usually ride with earplugs, hence the need to turn up the ipod and intercom, my co-rider can set her volume the way she wants. Good luck.