I don't recommend putting them together at all! Maybe supervise their "meeting" but remember quarantine. My QP and cockatiel still try to fight with each other and they've been cage nieghbors since February. Plus, if you're planning on keeping the new cockatiel, be sure to use the quarantine recommended for unknown or new birds before integrating them into your flock.

Is anyone around your neighborhood looking for a lost cockatiel by chance?