How long are they? I would just cut back the 2x about 18", and then tie in a section 6lb and then from that try to go to the 2lb. If the diameters are way off, you can either add in some more sections to help the transitions, or just use a triple surgeons knot to secure the material (instead of a usual double surgeons knot). You may end up with an ugly looking leader, but if your tippet is long enough it will cast ok and it will catch fish - and I doubt you'll be in danger of breaking anything off if your knots are good. Your knots will be at least as strong as the 2lb. tippet, even if the diameters are way off.