limitation between subtasks and related tasks

Hello!

I have a problem with some issues in Redmine.
My project has and issue (42165) which is related to another one (42166). Issue 42164 is parent of 42166.
When I try to put issue 42164 as parent of 42165 too, I get the next message "Parent Issue is not valid".
My question is, Could two issues be children of another one at the same time they are related?

Thank you very much