Two twin box springs are not the same size as a queen mattress. A queen-sized mattress measures 60 inches wide by 80 inches long. A standard twin box spring measures 38 inches wide by 75 inches long.
Placing two of them together would give you too much width and not enough length. Even though an extra-long twin box spring is long enough at 80 inches, placing two side-by-side would result in 76 inches in width, which is 16 inches wider than a queen mattress. However, two extra-long twin box springs put together are the correct size to hold a king-sized mattress.