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Here I am on my kick about people doing things the hard way. If you've read any of my numerous posts about why it is completely unnecessary for a solo boater to lift the boat over their head when loading, you can imagine that the same can be true for a tandem couple.
(Bear with me through the intro part here). My method for solo loading is to put the rear cross bar close enough to the back of the car that one end can be put up there while the other end is on the ground, or one end can be guided up there while carrying the canoe on the shoulders, then I step out from under it with one end on the rear bar and the other on the ground, so I can simply grab the other end and slide it up. I've done that with heavy aluminum canoes and fishing boats too. Clearly if your wife doesn't want to help load the boat, you should be able to do it alone if your rack is set up for this kind of loading (and there are about three other methods that work in the same fashion, one of which I'll mention below).
When I load a boat of the weight you describe here with the help of my girlfriend, who like your wife, can't lift her end of the boat overhead, so she simply holds her end of the boat at waist level while I lift MY end up onto the back cross bar. And if she couldn't hold her end that long, she could simply set it on the ground while I lift my end. From that point, it's just as described above, except that SHE pushes the boat up onto the roof. Once you've got one end of the canoe overlapping a bar, a child can accomplish the rest (the free end of the boat gets progressively lighter as the boat is pushed forward).
Now, if you can't put your rear cross bar that far back on the car, buy or build a loading bar that extends out off one end of one of the existing cross bars (If you lived near me, I'd custom build a clip-on loading aid for you). Use any of the normal one-ended loading methods (solo carrying or tandem), then once one end is up on that extender and overlapping by a few feet, you can go back and pick up the other end and shuffle it to the side so it's on the other cross bar. Then go back to the end that on the extender bar and shuffle it sideways onto the main cross bar as well. A side-extending bar can also be used at the rear cross bar for loading the boat from the rear, and though requiring a bit more boat-handling dexterity, the effort required is much less because as you slide it farther forward it becomes necessary to lift only a very small fraction of the boat's weight (just as when you can use your rear cross bar alone as your loading aid). But even when a side-extension bar is used the normal way, once the overhead-lifting part comes, you'll only be lifting one-third of the boat's weight, and in the case of this boat, that's just 20 pounds.
There's nothing wrong with wanting the lightest boat practical, but to assume a boat must be below a certain weight to allow overhead lifting is short-sighted. Just because many people DO use the overhead lifting method doesn't mean it's the only way.