starting application after su _another_user_

While I'm logged in as user_1, I'd like to start an application for user_2 but I get a D-Bus session error:

username_1@hostname:~> su username_2
Password:
username_2@hostname:/home/username_1> /usr/bin/kmail
(25123)/: Cannot find the D-Bus session server


KDE4 Troubleshooting documents a solution to it:

"If you get
KUniqueApplication: Cannot find the D-Bus session server
make sure you can access the display, e.g. type
xclock
then a clock must appear on the screen. If it does and the error persists, start your shell using
start your shell using

dbus-launch xterm

and start your program in there."