This is an old thread, but I had the same question and just figured it out.
Simply linking/attaching a card is what they considered once directional:
Link Card A, to Card B. This only attaches Card A to Card B and does not affect Card A.
Connecting the Cards or Relating them makes it 2 directional: The link will appear on both cards.
You can see some more details in this article: The Basics of Using Linked Cards in Trello
When you hit Connect Cards, you'll get another prompt to "relate". Choosing to "relate" will add a link from the connected card to the original card.
So if I add Ping 1 as an attachment to Ping 2 and hit connect/relate, then Ping 2 will also be added as an attachment to Ping 1.
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