A queen mattress is not the same size as two twin box springs. A queen-sized mattress has a width of 60 inches and a length of 80 inches. A normal twin box spring has a width of 38 inches and a length of 75 inches.
Combining two of them would result in too much breadth and insufficient length. Although an extra-long twin box spring is long enough at 80 inches, when two are placed side by side, the breadth is 76 inches, which is 16 inches wider than a queen mattress. Two extra-long twin box springs combined, on the other hand, are the right size to hold a king-sized mattress.
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