<p>Math/CS is both feasible and a reasonable idea in that if you’re good at pure math you’re likely good at theoretical CS too. Quite a few courses satisfy requirements for both majors. It’s not the most widespread thing to do – an average year it might be one or two or none who do it (in my nonscientific recollection) but there’s no huge reason not to do it. Plus, if you start with it and realize the workload is too much, you can always pick your favorite of the two and do that.</p>