What happens when in a multiplayer game i play "King Joffrey's Guard" And Already have "Queen's Guard" in play?

Since King Joffrey's Guard say: " After King Joffrey's Guard comes into play, choose and kneel up to 2 non-Army characters. ",

and Queen's Guard says: " After you trigger an effect that kneels an opponent's character, choose and kneel 1 character controlled by a different opponent. "

Do i kneel 1 or 2 characters with Queen's Guard?

What makes me confused is that Queen's Guard doesn't say "...effect that kneels 1 or more opponent's characters..."

You have triggered one effect (King Joffrey's Guard's response), so you get to kneel 1 character for Queen's Guard's passive. If the 2 characters you knelt with King Joffrey's Guard's response are controlled by different opponents, you can kneel a character controlled by any opponent.

I still don't get it from your second sentence.

If i kneel 2 characters from 2 different opponents with King Joffrey's Guard, do i kneel only 1 character with Queen's Guard, or 2 characters?

Let's say you have three opponents, Alice, Bob, and Charlie. You trigger KJG, and kneel one character each belonging to Alice and Bob.

Alice's knelt character lets you trigger QG against Bob and Charlie. Bob's knelt character makes Alice and Charlie eligible targets for QG.

Hence, choosing more than one player for KJG makes any player eligible for QG.

But you still can only kneel 1 character for QG, because KJG is a single triggered effect.