You sure?
I was concerned about that too!
Seems a weird rule to allow one player and one player only to be involved in dual competitions?
YNWA
See section 18.18 from UEFA rules (Top of page 17) Unless I'm reading it wrong (which is possible).
One player from the above quota of three who has played UEFA club
competition matches for another competing club in the current season may
exceptionally be registered provided that the player has not been fielded:
- in the same competition for another club,
- for another club that is currently in the same competition.
Furthermore, if the player’s new club is playing in the UEFA Europa League,
his former club must not have played in the UEFA Europa League at any
point in the current season.