I would also buy both bedding - King and Twins. The idea of the toppers is brilliant! We use these types beds when cruising. Never noticed much of a difference than from our King bed at home. We like our own space and do not use the middle - much. :) What we really like is while traveling in Europe, we have slept in two fully furnished Twin beds that are pushed together. You are next to your spouse sleeping, but have your own bedding and mattress. No worries waking your spouse when tossing and turning. Frankly though, if I were a guest in someone's vacation home, I wouldn't care what type of bed I was given. I'd just be happy to be there. As a married couple of 35 years, we can survive a few nights apart. I actually prefer having my own space sometimes...but don't tell the DH! I am glad that we do not have to modify bedding for guest. Since becoming childless when our last left the nest, we have 4 extra guest bedrooms - Double, Double, Queen and Twins. 2 bathrooms with double sinks in each. One set of bedrooms/bath are in a downstairs wing, while the other set is upstairs.. Both completely private. Something for everyone. :)

Eons ago when we first married, we bought two twin mattresses that were put on a swing out frame at the salesman's suggestion. We knew we would be spending a lot of our first years moving from one apartment to another and using the elevator to move. (This was when kings were new and elevators did not necessarily accommodate a king mattress.) He said that two twins would be easier. It worked well. The twin frames were on a special metal frame that went across the top end of the beds, but you could use twin bottom sheets and the frames would swing out from the together position to let you put the bottom sheets on. Then we slid them together and used a top king sheet. As broke, eloped newly weds, I just sewed twin flat sheets and twin blankets together at first from what we had for the top sheet and blankets. It served us well for many years. When we were finally able to splurge, we bought a king bedspread to make it all look nice.

I have a split-king Sleep Number where the "twins" can be raised or lowered at both the head and the feet, with motion, too. So far, I've only purchased the Sleep Number fitted sheets because I bought several sets on sale. The Sleep Number brands have a separate extra band of elastic that stretches diagonally across each corner, in addition to fitted corners. Regular fitted sheets don't come with that band of elastic. However, if you have a sewing machine, you could attach your own band of elastic to non-SN fitted sheets. For sizing of the twin, you want Twin XL or Twin Long, depending on the wording of the brand of sheets. For top sheets, blankets, duvet covers, coverlets, etc. I buy regular King size from any store.

Trust me on this; a while back we stayed at my mother's 2nd home between selling our home & purchasing another. She had two twin adjustable beds that you would hook together & make a king bed. While the frames & box springs stayed together we were up & down constantly pushing the mattresses together & it was very uncomfortable to be toward the middle. We used both king mattress covers & sheets; we had to adjust them & refit them when we pushed the mattresses back together. This has been a while but would never do that again. Maybe it is different now but that is my experience.