We have zipped two singles together in the past, but one of you gets your face covered by the hood of the mummy bag, nomatter how much you tuck it away! You would need one with a left zip and one with a right zip, I believe some of the more expensive bags are available in both types.
the best thing is to zip tow singel bags together they are warm but if you are still cold you can always cuddel up together if this fails you can then nick more of the covers lol
We have got 2 mummy type singles 1 with a left hand zip and 1 with a right hand zip and they can then be zipped together if we want. Best of both worlds!