UEFA regulations however can clear a player to play for two clubs in Europe in any given season, based upon an exception which applies to participation in the qualifying rounds.
Rule 18.07 states that if a player features in the first, second or third qualifying rounds of either the Champions League or Europa League, he can play for another club in either competition from the group stages onwards, provided his former employers do not reach the group stages themselves.
If a player is involved in any of the aforementioned qualifying rounds for a side which then goes on to play in the group stages of either UEFA competition, he is not eligible to play for another club in Europe in that season.