Okay, I have a really unorthodox suggestion for couples who have mattress issues. DH and I were in the market for a new mattress after just about 5 years on our supposedly "no flip" mattress. Big holes where each of us slept, lump in the middle, could barely move without the whole bed shaking. We both shift positions at night, and each time it woke the other up. Talk about no sleep!
We tried out different mattresses -- either in store or took note while staying at others' houses. We quickly ruled out pillow top (made our backs hurt), memory foam/latex (whole mattress, combo and mattress topper) as they made us super hot at night -- didn't even try the number or tempurpedic one because of cost. In desperation, until we found a permanent solution we decided to go ahead and dump our old mattress, and moved the two basic, firm twin mattresses we had on our guest room daybed into our room. We just put the two mattresses on top of our king box springs. Oh..my..gosh. Best nights of sleep in YEARS. When one of us moved around, the only way you'd feel it is one tugged the blankets too hard. The mattresses were a little stiff, so we just put cheapo egg crate on each covered with a padded mattress cover, and it has been heaven. A few people I've mentioned this as an option to sometimes look at us weird, like we must be on the verge of sleeping in separate rooms or something. But it really isn't any different than having a whole mattress, apart from sleeping better. Now, if you are a couple that like to cuddle the whole night, or you have children that sleep with you, this won't really work very well unless you like sharing a twin mattress all night long.
If you do like the idea, but want to make it more like a single, seamless mattress, they do sell these little straps that hold the beds together. You then can put a padded mattress cover over it to prevent feeling the ridges in between, top with a regular king sheet and voila! If you've gone on a cruise and shared a bed, this is what you've slept on. We did try this, and found that while it was still better than the single king solution (we've had a good nights sleep on every
Disney cruise!), it still wasn't as movement free as without the connectors.
FWIW: the twins do have to be extra long length to fit on a regular king box spring. They still end up being so much cheaper than buying a regular king mattress. I know sometimes going with the budget option means compromising for the less than ideal, but this has not been the case for us.