We've done the concurrent thing a couple of times, with different kids, just to make sure we weren't leaving gopher holes that might break an ankle later. Once with an elementary grade skip over fourth grade math, and most recently with precalc and calc I.
And sometimes a school/district that is smaller, or isn't known for it's high-achieving programming can actually be better for learners in the tails, because they may turn out to be more flexible, since they're not busy trying to handle large numbers with similar needs. At the small school I attended as a child, my alg I teacher made a similar recommendation to skip to alg II/trig about a month into the school year, which my parents agreed to. A couple of whole grade accelerations among my extended family originated from teachers in such schools. While they may have fewer financial and opportunity resources of certain kinds, they can have equally fine (or better) human resources, with less of the pressure on the system that comes from having to gatekeep those high-demand resources.