I don't know what you'd call this type of relationship

there are 2 tasks related to the same problem, but different isses

Create a new parent and assign both as children so both tasks are now "peers".

In this case we have a new task for nothing

Use some other document like Phriction to list them.

We use Phriction only for documentation, not for tasks.

Mention the other tasks in description.

This works, but you have to do it in both tasks, and you don't have a nice Task Graph.

We have tasks which takes more then 6 months to solve them. We create subtasks for this, and subtasks of this tasks and so on. At some point there are tasks related, but different issues. They also have different parents. In order to see a nice Tree view, it would be cool if we can connect them between them.

In this case we have a new task for nothing

Container tasks are very common in Phabricator, I'm not sure what problems you anticipate them causing.

We just want to connect 2 tasks, not to create one more task. If we add more tasks then the Task Graph get more complex.

Then I'd recommend forking Phabricator to provide the functionality you seek.

Essentially, if there is a way to accomplish something in Phabricator, but it's not an install's ideal solution, we're not likely to add a second means of solving the same issue unless it meaningfully solves something new. We have 2 developers and thousands of tasks competing for upstream resources - so prioritization is difficult for tasks like this.

I understand the situation. I will have a look myself.
Thanks for your answers!