Card mirroring with Butler
Butler allows for basic card mirroring, or in other words, make the actions on one card automatically perform changes on another card. This is typically done on cards that are copies of each other, perhaps living in different boards.
Full card syncing/mirroring
Please note that trying to keep two cards perfectly synchronized with Butler is not recommended. In addition to the significant number of rules that would be required, it can potentially cause commands to loop infinitely when the action of one command triggers another.
If you need full card synchronization between boards, there are specific Power-Ups and services that are designed for complete one or two-way syncing of cards: https://trello.com/power-ups/search?q=sync.
Cards can be linked together as attachments or as checklist items, which can then be found by Butler using the actions in the 'Cascade' tab.
Then, we can mirror certain changes in one of the cards to the other linked card.
If you have questions or trouble syncing cards, feel free to reach out to the Trello Support team for help!