You will notice I have moved your question to an existing thread. If you look back through the posts, you should be able to find a lot of useful information. In particular, have a read of my earlier post: Adjustable beds - #8 by meltam.
We have two separate XL single beds pushed together to make a king single. The downside is that it is very hard to by bed linen in that size. The upside is that the beds are the same length as a queen, but each of us has a bit more space, and we can move one bed at a time on those occasions we need to clean where the beds are located. Also another big plus is if anything ever goes wrong with a bed, the other bed is still fully functional.
If you get a queen frame with two single mattresses it will be much harder to move the bed, and if anything goes wrong, the whole bed is out of commission. On the plus side, bed linen will be easier to find.
