Baseball and football is the most common combination that two-sport athletes have found success in. It makes sense. There’s only a slight overlap in seasons. In any levels lower than the professional ranks, there’s basically no overlap. Running, throwing, and catching are all skills that are important in both.
While there’s a decent-sized drop off from baseball/football, that’s not the only sport which has seen someone at least get drafted in both. (Shout-out to baseball Hall of Famer and NHL draftee Tom Glavine.) The next most common cross over you see is probably baseball and basketball.