No, what Isiolia is talking about is when you first start killing them. The mamool DRGs are on timers just as PCs are. They have a cooldown timer on Call Wyvern. (I would assume 20 minutes just like a PC) Oftentimes the mamool are left alone when no one is meriting on them and their cooldown timer passes. A party comes along and starts killing them, kill the wyvern first and the mamool can summon another immediately, just like a PC DRG when their wyvern has been out a while and the cooldown timer has reset, because the mamool's cooldown timer is usually up.
Once they start repopping though you will never get two wyverns again unless you let them live long enough for the cooldown timer to reset.
XI Wiki


