A queen mattress is not the same size as two twin box springs. 60 inches wide by 80 inches long is the size of a queen-sized mattress. 38 inches wide by 75 inches long is the size of a typical twin box spring.
Combining two of them would result in an excess of breadth and a shortage of length. Even if an extra-long twin box spring is long enough at 80 inches, placing two of them side by side would make the mattress 76 inches wide instead of the queen mattress’s 16 inches. But when combined, two extra-long twin box springs are the ideal size to support a king-sized mattress.