Certainly you can do this. I have done this many times in historic houses where neither a King, nor a Queen, will fit up the stairs. I would not do it based on expecting longer service life out of a bedding set, however. The X-Long Twins will break down at the same rate as a standard King, though you are correct that they're easier to flip that way.

Shifman makes a "Zippered King" which has two industrial zippers on each side of the matt. When zipped together they are virtually indistinguishable from a standard King. I've been after Royal-Pedic to do the same thing, but they have resisted to date saying not enough customers have asked for this feature.

If you do go with a pair of Twin XL's, what you can do is buy extra firm hotel-grade units, put a 3" Latex topper over them, and then tie-in all in with a high-quality mattress pad that has spandex in it. Done like that, you'll never know the matts are separate.

Duane Collie

We have two twin XL mattresses on our king bed and much prefer them to our old king mattress. We topped them with a very soft faux featherbed (high loft fiberfill) that makes them extremely comfortable. One benefit is that movement isn't telegraphed from one mattress to the other, so we're less likely to wake each other up when keeping different hours.

Just to be clear, the ones in Europe are, I believe, on one box spring. Still it cuts down on movement and the matresses are easier to flip.
You can still use your current box spring on your king bed frame and just replace the mattress part.

Your right it will help the mattresses last longer because you have less "space" for the innerspring units to traverse. It is a good idea!

Its not the springs that break down in a bed, its the over and under layments, most of which are made of blown polystyrene foam. Literally, the foam crumbles internally. Expect no longer service life from this arrangement.

Duane Collie

Also worth keeping in mind all the different kinds of beds out there and investigating the different kinds of wear and tear they'll experience. This will also have to do with how you sleep.

I'm fairly recently married and our bed is thus far lasting well - but it is such a hassle to flip. We've taken to flipping our sleeping position instead of the mattress at times since we've had it. I doubt this is quite as effective, but better then nothing.

When we were newlyweds and would be moving from apartment to apartment for a few years, a salesman suggested we buy two twin mattresses and they sold us a frame that joined the two. We used twin fitted bottom sheets and a king top sheet. The frame was on wheels, and when I changed the sheets, the two halves rolled apart at the foot (staying connected at the head) to let me get between the mattresses to change the sheets. That worked well for all our nomadic years.

Ya! its a good idea.You can use it in future comfortably.

Here is a link that might be useful: Twin extra long bed