Originally Posted by Michel Gagnon
(...) you will be much more stable with decent fore/aft weight distribution.
I agree with the concept of the weight distribution, and if you buy regular front and rear panniers, you'll be able to use them on a regular non-tandem bike as well. I'm not sure if 84L panniers fit on a regular pannier, but it seems huge!
As for Arkel, they make great products. All my panniers are Arkel: T28 in front, Dolphin 48 in the back, and a large handlebar bag. I also have their tail rider (the Pelican I believe) and they all perform flawlessly.
If you want a truely waterproof bag, I'd suggest the Dolphin bags. They're Cordura on the outside, and the inside is covered with some sort of rubberish material that's quite similar to the material used outside of Ortlieb bags. I really like them because they are truely waterproof and, unlike the Ortliebs, have an outside pocket and a couple of mesh pockets.
As an added bonus, they're made in Quebec. You might not care though, but I do as I live here
If you go for the simpler (and cheaper) models, the T48 (is it 48?) in the back and the T28 in front, I'd strongly recommend the raincovers, which are really nice to have when it's pouring. Although after a full day of heavy rain some water will seep in (especially if you don't have fenders).
Ultimately, I find the Dolphin like of bags to be the sweet spot in term of price/performance. One caveat, they're only available in blue, not that I care, but you might.
One last thing about Arkel bags is that they have a lifetime warranty. You don't even need to have the sales slip. They'll fix them as long as the Arkel label is still attached!
Have a nice trip!