I have THE most uncomfortable bed in the world. I feel so ripped off (2 years ago). The worst part is that it is two box springs in twin size and a big soft space in the middle. Is that normal???
Mine is 2 twin box springs, but over time, the mattress seems to get a ridge in the middle because it doesn't compress over the edges of the box springs where they meet in the middle. Maybe it would compress more if we actually slept on that part. I actually like sleeping in the middle when I can, because it feels comfortable to me. But if it is soft in the middle, that doesn't sound right. It doesn't sound that your box springs are big enough.
There shouldn't be a gap in the middle. I think I'd be inclined to try putting some plywood over the boxsprings to make sure the whole thing is big enough to support the mattress, and see if that improved it. (I take the cheap 'let's see if this works' approach though.)
Our king size mattress perfectly fits over the box springs, with no gaps anywhere.
I think I may have made a mistake. We don;t have a box spring, if that is a second bed that the bed goes ON. I just have the mattress. So, the mattress have two twin mattresses and a soft space in the middle that is about the size of a twin.