Each leg of an award can only be: AS + 1 partner.
Thus, ATH-AMS-SEA is not possible because ATH-AMS is operated by KL and AMS-SEA is operated by DL, making 2 partners.
You could however find an award ATH-AMS-YVR-SEA, as ATH-AMS-YVR would be operated by KL and YVR-SEA by AS.