First of all, they'll be a lot closer in size to each other than your picture would indicate. Second, the linear guides are almost always bolted to a steel or aluminum plate, with bolts every 4-6 inches. Third, linear guides carrying loads in any direction other than along down (in your pic) should be used in pairs, which would eliminate the twisting you're worried about. Fourth, the linear guides can carry much higher loads because the balls ride in a ground race, as opposed to riding on a round shaft with an infinately small contact point.