Will 2 twin box springs fit a full size mattress?

No, two twin box springs will not fit a full-size mattress properly. A full-size mattress measures 54 inches wide by 75 inches long, while a twin box spring typically measures 38 inches wide by 75 inches long. While the length matches, the combined width of two twin box springs (76 inches) exceeds that of a full-size mattress, which would result in an improper fit.