We just used to use 2 single ones and zip them together so that one was on top and was underneath ,we found it fine ,didnt get tangled and we were greatful of the extra layer :)
I use two flannelette double sheets, stitched together along the bottom and halfway up each side. They make a brilliant liner and are very cosy. The children have single sheets folded in half and stitched, but they don't work as well and they do get tangled in them.