While the best thing is complete disassembly and thorough cleaning, I have been successful in resolving similar problems with Mafac Racers by lubrication and light cleaning. They're pretty robust. By design they are self-centering in use so your observation indicates there is enough friction somewhere that the forces are moving the one arm more than the other. As suggested, straddle cable and straddle cable holder, and pivot points are easy to look at, lube and perhaps solve the problem without disassembly (until you have enough time to "do it right").

As a reference, I just checked the Mafacs on my Peugeot: slowly operating the lever, one arm moves a bit first, the second arm starts to move a bit as well but slightly more slowly, the leading edge of the first arm's pad just touches the rim surface and seems to "pause" a bit, the other arm moves slightly more rapidly until its pad just touches the rim, then both arms move evenly as the brakes are applied.....