I'm not sure about that idea of trading the trailer back and forth. One person will be struggling while the other is bored with the slow pace. And the trailer will be really really heavy, with both people's stuff in it. Also, if you get separated or decide to separate, you are without your mid-day neccessities, like rain jacket and tools. I think it would be better to both carry your own personal gear, and share or trade any shared gear between you.

For gear lists, go to crazyguyonabike.come and look at some journals with gear lists.

I have a bias against trailers, but if you do decide to use a trailer, you just need any bike that fits you, has strong wheels and low gears. I can't emphasize low gears enough.

The "usual suspects" for touring bikes (that are good for rack/panniers) are:
Novarra Randonee (REI house brand)
Surly Long Haul Trucker ("LHT")
Trek 520
Fuji Touring
probably a few more.

You can also stick a rack (or trailer) on just about any old hardtail or rigid mountain bike, if you need to save $. You're looking for low gearing, solid wheels, and long enough chain-stays that you can fit panniers on a rack.

Have fun.