Neither of those bed sizes doubles to make a king. A standard or eastern king mattress is the same size as two twin XL beds, meaning it’s 80 inches long and 76 inches wide. Whereas two twins would form a nearly symmetrical square of 76 inches wide and 75 inches long. And two fulls would be 108 inches wide by only 75 inches long. All this means if you’re looking for a split king, you’d better make sure you’re getting twin XLs to go in the standard king size bed frame. No other standard size will fit.