I understand that this may be of little use since you already have the USB headsets, but...
Why use USB headsets at all? If you're using a PowerBook, then you have an analog line-in.
If you're looking to record two people at once, then why not get an inexpensive mixer (let's say the Behringer UB802 for US $50) and then 2 non-USB headsets?
Plug the headsets' mic cord into their own channels on the mixer, then the headphones into a y-adapter on the headphone jack.
Then, take the line-out from the mixer and connect it to the line-in on the powerbook.
Set your System Pref to listen to the line in and you're good to go.
Audio Hijack Pro and Sunflower are great, but I think you could get better results just using simple hardware. Plus, you could also record Skype with the setup I mentioned above by simply using the headphone jack on the powerbook and plugging it into a 3rd input on the mixer.
You could get a couple of decent Plantronic headsets for under $20 each, plus the $50 mixer and necessary cables for all under $100 for a usable setup. Upgrade when you have more money.
I use basically the same setup except with "broadcast quality" gear for a couple of the podcasts I produce. Check them out and let me know if you have any questions.
http://www.thepilatespodcast.com/podcasts/podcastarchive.html (the one I produce for my wife, Heidi)
http://www.birddogsforever.com/bird_dogs_forever/podcastepisodes.shtml (the one I produce for a client. I used my setup for the audio only episodes.)
Good luck!
Jon
TreeLine Productions
Fort Collins, CO USA
