Most folks store their canoes upside down, even indoors, for a couple of reasons.
Critters (mice, squirrels, chipmunks, etc.) often find a right-side-up canoe a handy place to nest and/or defecate, and chewing up those tasty, crunchy ribs and decks is easier for them when the canoe is right-side-up.
People often use right-side-up hulls as ad hoc storage bins -- for pfd's, paddles, bags of fertilizer, garden hoses, etc., -- of course, neither you nor I would ever do something like that
. And when that happens, the critters are even happier than with an empty hull.
Further, it is usually much easier to build a rack for upside down storage -- the chief concern is to make the storage brackets horizontally parallel, so as to not induce any twist to the hull.
I also find it hard to imagine a sling rack for 2 canoes that would not be quite awkward to use.
If a rack is also to be used to display, or to work on a canoe, a sling rack may be more useful -- but a rack for two canoes would not be an ideal place to work on a canoe.
Using a length of carpet to support the entire length of a canoe seems not just overkill, but also not really effective, given the complex shape of a canoe hull. A carpet sling would give even support to a length of round pipe, but I'm not sure what it would do with a canoe hull -- it might only support the wide center of the canoe where the carpet would be stretched, while offering little or no support at the narrow ends, where I would think the carpet would be slack. Slings, just a few inches wide, are usually made of canvas, seat belt material, old fire hose, toweling, or some such thing. Two fairly light slings, placed about a third of the way from bow and stern, are more than adequate for almost any canoe.
