After testing it for about 10 minutes in Kuftal I have to say I actually doubt Excalibur's Additional Effect overwrites the effect of En-2 spells. I have been fighting Robber Crabs about 10 minutes removing all forms of Double Attack in order to make sure I was only seeing attacks of which would normally be effected by En-2s and in the 10 minutes I experienced not a single HP Proc. I am only using a lv85 Excalibur, however I would doubt that makes much of a difference, so I have to say for now that this is false until shown otherwise.