Since you have a dual tank set up, I would be surprised if both floats in both tanks were stuck (still possible). Could be a sending unit, but then again you would have one in both tanks and for both to be bad (also highly unlikely. Deffinetly electrical problem, but before you start dropping tanks and replacing guages, I would do what gfw1985 suggested with grounding the guage. If it pegs full then not the guage or a fuse. Tace the wire from the guage to each sending unit. I am guessing you are grounding out since neither sending unit in either tank is working. (Check where the new guage was installed.