Simply add one card as an attachment to another.
The attached tasks will be opened on top of the linking ones, for quick navigation. Go to Attachments and use the Link to a different task option.
Use the Task Dependencies Power-Up.
It has been designed to help manage complex jobs. Large tasks can be broken down into pieces, arranged in readable structures, showing clear dependencies between them.
You may also use tags to link 2 or more cards.
Just apply the same tag to several cards. You will then be able to filter for this tag, and quickly locate tasks that contain it: