I'm looking for a used tandem myself. My gal has decided that she might enjoy bike touring with me and wants to do the tandem thing. From what I've read, I think I'm going to stay with panniers. I use Lone Peak P-99' and P-100's—their two smallest panniers. For tandeming I'm going to go with a new set of P-500's to accommodate the missus's extra gear (I'm pretty sure it'll be more than I carry) and drop the P-99's. As mentioned above, I use a dry bag on top of the back rack for the sleeping gear and put the tent on top as well. Works out pretty well. Guess I'll see how it goes on a tandem.
I decided against a trailer. There's the extra weight of the thing itself and the necessary spare parts to carry. Also, I have to get used to the extra length of a tandem and a trailer would make the whole rig longer still. You never know, I could change my mind, but I think we'll be pretty happy with a four pannier/dry bag set-up. Good luck to you folks. Let us know how things go for you.