if you buy a bed 'doubler' to make two twins act as a king, you MUST have twins of the same size and height. otherwise it's not a pleasant bed experience. i say this from using the bed doubler for guests and encouraging their input. i got one that had a piece that fitted over the 'seam' where the two beds came together. finding that not so comfortable for guests wanting to snuggle (which was the point after all), i bought a good quality foam topper to fit over the two mattresses when it was being used as one bed. fitted sheets are a challenge ~ i think i ended up using california kings. it was a lot of trial and error. one enthusiastic couple had the doubler open up and the beds moved apart in the night. and it was never my intention to embarrass anyone and i tried really hard to avoid issues with the bed(s).
i believe there are others on forum who use the doublers in their b&b's.