Using 'stuff' to send characters from one account to a second account's session

If I create a session under one user's account:

	screen -d -m -S myapp -t myapp /bin/bash

I am (having set multiuser on and acladd) able to access it from a
second account:

	screen -x user1/myapp -S myapp -p myapp

However, I am not able to figure out the right syntax to use stuff to
send text from the second account to the first account's session.  This
does not work to send text, instead it only attaches to the session:

	screen -x user1/myapp -S myapp -p myapp -X "shutdown"

What is the right syntax to send from the second account?

Regards,
/Lars