I have seen an XCX tire in person, and I am pretty sure it would fit. Definitely the 29x1.95 would be fine in the front, the only worry would be whether those small side-knobs would rub in the rear. 29x1.75 should have no problem. If you were buying from brick & mortar (and they had these rare tires in stock), I'm sure they'd let you test the fit before purchasing. Another possibility, you could order a single 29x1.95; if it fits on the rear, fine, order another; otherwise swap it to the front and order a 29x1.75. But of course, that's two shipping charges. You could put XCX on the front, and V12 on the rear.

If you do end up determining for sure whether XCX in 1.95 fits in the back of a CrossCheck, please do come back and post some pictures of the clearance!