For the 2nd time, here are the UEFA guidelines that I had posted earlier today
Can a player play in UEFA competition for more than one club in the same season?
As a rule, a player may not play in a UEFA club competition (UEFA Champions League and UEFA Europa League, excluding UEFA Super Cup) for more than one club in the course of the same season.
However, a player can play for one club in the first, second and third qualifying round, and/or the play-offs, of the UEFA Champions League or UEFA Europa League, and then represent another club from the group stage of either competition.