As far as I am aware,
In EASA countries, you can not work for more than one AOC holder at the same time. Everytime you want to commercially operate an aircraft on another AOC, you are obliged to do an Operators Conversion Course. You CAN NOT mix and match at will.
Be very careful, some "self employed" guys are in breach of EASA regulations and can get in serious trouble, as well as the companies they fly for.