@ A Twig
No, if the effort, challenges and responsibilities are going to be very different, I'd set this out in the job descriptions, and set the two positions at different points on the pay scale. They may start at the same company, at the same time, working the same number of hours, and both do a "good job," but they would not be working the same job. It might be as simple as paying a higher hourly rate for the unsociable hours of the evening shift.
I'm going to regret this when someone pulls out the relevant piece of legislation that shows my solution to be somehow illegal, but it's surely a fine principle?