Unfortunately, twin box springs will not work on Eastern king or California king mattresses. A regular twin is 38X74 inches long and the long twin is 38X80 inches long. A Cal king bed is 72X84 inches. Two long twins together would be 76X80 (too wide and too short.) You need to buy a box spring for the specific mattress. A king box spring will likely come in two pieces, but it shouldn't be confused with twin box springs.