King Sized Artisan Base, Zip and Link, Pocket Sprung Unit

I am unable to move a king sized mattress or divan bed up the stairs, the access is too small.
Current King sized mattress is a The Viscount from British Bed Company. This mattress was bent over, with straps, length-ways, by removal men. The mattress, on a slatted bedstead, has a mound in the middle and slops off to the edges on both sides. it is also uncomfortable, feels like the springs are digging into both of us.
To spread the costs I was thinking of the Divan mentioned in the subject line, followed by a zip link King mattress later on. I would be using the current mattress in the meantime (it can't be any worse!)
* I assume the Divan has the 'True Edge' all the way around both halves of the zip-link, effectively like two single beds. Will this continue to cause a mound down the the centre of the mattress, as there will be two edges right in the middle of the bed?
* Is this even a good idea? Will two Divans and one mattress, or later two mattresses, linked together, always just feel like two beds pushed together? I.e. with a ridge down the middle of the bed, not like a shared king sized/
Kind regards,
Sam