JP,
As a side note, I've carried Old Town Pack canoes on 185's, Cubs, 206's and Husky's a lot. At one point, I think we had thirty of the things (black bears really love to eat them, so their numbers tend to diminish annually).
One pack canoe on one side of any of these aircraft, flys just fine, with little noticeable effect.
Two pack canoes on any of these airplanes really changes the way the airplane flys. Much slower, and MUCH less stable, side to side. A lot of drag.
Now put two of these on one airplane, and load the airplane to gross weight, and you have a real handful. Stability is REALLY poor, airplane doesnt want to climb, etc.
So, when experimenting with these things, do just that, a little at a time. Don't ever make any assumptions regarding how an external load will fly, even if you did the same load yesterday.
There are some deeply moving religious experiences involved in moving external loads.
MTV