In reply to RhysRolfe:
apologies, my maths isn't what it used to be
.....
more dumb questions: are the mats self inflating foam things? karrimats would simply overlap each other a bit, but thermarests might just push the walls out a bit. can't see a huge problem in that, would expect a bit of leeway/flex..i'm sure MSR etc would have accounted for the width of two 'standard' mats.
i suppose the only real way is to try it in the flesh...over to you, the great umwashed UKC public!
(PS have you tried asking on outdoors magic as well?)