I am looking at buying a double sleeping bag that converts to 2 singles. I have seen an ULTRACAMP bag which seems to do what I want. It is filled with 400gsm.
I work for a company which supplies these. Not sure if I'm allowed to comment - as I'm obviously biased! If you check Amazon.co.uk there are a couple of reviews there. Also if you check out the ebay feedback on the company selling the sleeping bags you will see what others make of them.
Hope that was of some use. Sorry if I've broken any rules by commenting on this.
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