"Chain Disappearance" can be activated even if a monster with 1000 ATK or less and a monster with more than 1000 ATK are Pendulum Summoned simultaneously. (In that case, only the monster with 1000 or less ATK is banished by the effect of "Chain Disappearance", and the "your opponent banishes all cards with the same name as that card(s) from their hand and Deck." is also applied.)
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
You can activate "Chain Disappearance" in response to your own Summon of a monster, in which case your opponent still removes all cards with that name from his hand and Deck.
↑Only "Chain Disappearance"'s controller's opponent banishes all cards with the same name as the Summoned monster from their hand and Deck, not both players.
↑Konami Judge Program Rulings are considered unverified, as their source(s) are not publicly viewable. They are not confirmed as applicable to the TCG until they have been officially published by Konami.