Hi Kev, 
 
Thanks for your question. 
 
Where there are 2 separate mattresses you will always know that they are 2 completely independent entities. 
 
As such, a mattress topper is the only realistic way to get around this. 
 
A King Size mattress measures 5FT x 6FT 6IN whereas a Single mattress would measure 3FT x 6FT 3; with 2 of these side by side totalling 6FT x 6FT 3IN. 
 
It might be worth noting that an Extra King Size Zip and Link mattress (Where they can be used together or apart) would give you 2 mattresses measuring 3FT x 6FT 3IN.
 

 
To discuss suitable mattresses we would need to know your preferred feel in the top layers as well as sleeping habits as these can impact the mattress choice.
 
At your body weights you will need a dual tension mattress; You will need a Firm spring tension and your wife is borderline between a Soft and Medium spring tension. If she prefers a bit more give then choose the Soft or for a bit more support choose the Medium. 
 

 
In regards to toppers we only retail products suitable for use with Foam or Latex mattresses so not generally something we recommend for pocket sprung mattresses such as our Origins and Artisan ranges. It might be worth looking at a company online called Soak and Sleep; They have a much wider range than we do.
 
Please come back to me at your earliest convenience to discuss suitable mattresses or if you do have any further queries on 0161 437 4419.
 
Kind Regards, Lee.