We have two of the "ThermaRest DreamTime XL's" got them back in 2008 they can be fastened together with there embedded straps to make a QueenSize bed, although £119 each at the time, I consider them money well spent.
And for our fortnight campings in the South of France, during July they have been sublime.
We have an Aldi one. My daughter sleeps on it and I tried it the other day and it was very comfortable for me too! Going to get another one when Aldi next have them in again.
Not sure if they do a double but couldnt you put a cover over them?
Have used a Thermarest when I used to backpack and very good they are too. I now use a Vango 7.5cm as it goes in the car and I've found that very good too. I've even converted my wife to a SIM from an airbed (does that bring weird images to mind?)and she gets on well with it.
------------- "People sleep peacefully in their beds at night
because rough men stand ready
to do violence on their behalf."
Outwell elegance 10cm, which we upgraded from the aldi ones, which were an upgrade from some hi gear ones (don't go there!).
We looked at the outwell double 7.5 cm but the pack size was huge.
------------- 2013
Easter - Thorpe park tin tent
May bank - Saxdale
Whitsun - Nantcol
June - Studfold
July - Priests house, Barden's back garden ;-)
July- 4 vents Paris for euro Disney
Aug - Saxdale
Aug- high straggleton farm
Aug BH - Studfold
Sept - Stratford
Oct - Masons
NYE -saxdale (eeeek)
We've got a vango double 5cm SIM. Its much more comfortable than an inflatable mattress which I don't really like at all, But also much much more comfortable than the 2.5cm single I have. Wouldn't be without it.
We have two Costco ones (about £22 each if I remember correctly). Normally I put a double sheet over the top to hold them together, but with a baby in the middle, you could try self adhesive Velcro? I also got the aldi sims, and double them up :) 10cm of pure comfort!
I never really got along with SIM mats. We had the costco ones but I found that my hips and shoulders were pressing on the floor and I was sooooooooo uncomfortable on it.
Did try the Gelert 10cm double one in a shop which seemed to be really comfortable but we had an issue with pack size so ended up back on airbeds.
In essence, if at all possible it really is worth trying to find somewhere where you can try them out. Interestingly, I know when I tried them there were differences between brands (5cm styles varied massively in comfort). Lots and lots of people love the SIM's though and sleep comfort is such a personal thing. I now know if I was considering SIM's again in the future it would have to be 10cm for me.
Funnily enough my 7.5 double Vango SIM is more comfortable than my 10 cm Gelert SIM. You can see (when they're side by side) that the 10 cm is markedly thicker, but then maybe it compresses more when lain on.
Have a look at the Airic - following reviews on here we bought a couple, which were great. It's only the creaky bones of old age, and very hard ground in France made us get camp beds (with the Airic on top). 75mm seems about right.
We have a couple of old thermarest mats 3.8 trail lite, very good. Bought 2004, used lots.
The kids have them now and we have Mountain Equipment 3.8 from Cotswold at 45quid each. Also excellent. Pack tiny, easy to roll up and comfy. See the reviews.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.