Apparently, if the same shell launches multiple ssh connections to the same server, they won't return after executing the command they're given but will hang (Stopped (tty input)) for ever.

This is common behavior of concurrent access to TTY. The whole process is already backgrounded and when it tries to write the output, it is not allowed to access TTY and receives a signal (SIGTTOU), which is not caught by bash process and therefore the default action is performed (Stop).

The -n option as explained in the other answer or redirection of the IO to some files will help you. Not sure if there is something more to describe, but if so, please clarify.

Apparently, if the same shell launches multiple ssh connections to the same server, they won't return after executing the command they're given but will hang (Stopped (tty input)) for ever.

This is common behavior of concurrent access to TTY. The whole process is already backgrounded and when it tries to write the output, it is not allowed to access TTY and receives a signal (SIGTTOU), which is not caught by bash process and therefore the default action is performed (Stop).

The -n option as explained in the other answer or redirection of the IO to some files will help you. Not sure if there is something more to describe, but if so, please clarify.