So generally speaking if ropes are different diameters for an abseil you'd use the double fisherman's, but if the difference in diameter isn't very much would an overhand/double overhand knot still be ok to use?
If in doubt i'd use the fisherman's, but having jammed ropes on a multi-pitch ab is not an experience I want to increase the likelihood of!
For example I have a 8.8mm DMM spectrum half and looking to pair it up with an 8.5mm DMM prophet