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AFAIK, no U.S. carrier allows sharing of unlimited on iPad and iPhone if you're trying to maintain the plan you're on now.
If you want to tether, they make you drop unlimited, for instance. There are jailbroken people who secretly tether phone and tablet, but you'd have to get specifics from them, and if AT&T finds out, you risk losing your unlimited. (They basically look for surges in data use.)
There are new bucket data plans that allow sharing of data among devices, but they're so expensive compared with any grandfathered unlimited plan that there aren't many scenarios when it'd be worthwhile to switch plans.
I have grandfathered unlimited with AT&T for phone as well as iPad, but I've been paying for them separately because there's no blending of plans allowed.
How do you grandfather an account?