I don't know the 500, but I'm running both on my 650 whithout any problem. But since the 650 have got an electronic ignition system, that might explain why it works. On a points ignition I would belive the double resistance will be a bit to much.
You can increase the power to the coils though, by power feed the coils via a double automotive light relay. It's an easy modification, and you will ge a noticeable better spark.