I want when I discard a card, another card in other place is discarded.
Not one given card, but a card in a given place.
Just to clarify, let me state what I understand by “discard” in VASSAL. Your “deck” and the discard pile are both decks. However the latter starts out empty. In your Card prototype you have a “return to deck” trait. You make the menu entry discard, pick Ctrl-D as they key command and select the discard pile as the destination deck.
So to implement what you want, the first step is change that “Ctrl-D” to “discard_this_particular_card_here” or something like that. You also remove the menu command. Then you add a Trigger trait, which must go before the “return to deck” trait. You make this trigger trait have the same Discard/Ctrl-D as the “return to deck” trait had previously. And you make the trigger output “discard_this_particular_card_here” (and you have to make damn sure you get it exactly the same, and no you can’t copy and paste).
For the second step you can add whatever you want to the trigger. You could a add trigger that output “discard_that_very_naught_card_over_there” and which listened to something your first trigger output. You also need to put something in the “Match properties”. Or may be the second trigger should actually be a global hotkey. You probably need to experiment a bit. I would go slowly using “report action” traits to make sure you understand each step.