The packages `systemd` 226-1 plus `dbus` 1.10.0-3 now launch _dbus-daemon_ once per user; all sessions of a user will share the same D-Bus "session" bus. The _pam_systemd_ module ensures that the right `DBUS_SESSION_BUS_ADDRESS` is set at login.
This also permits dbus-daemon to defer to systemd for activation instead of spawning server processes itself. However, currently this is not commonly used for session services (as opposed to system services).
_kdbus_ will only support this model, so this is also an opportunity to iron out some bugs and make a future transition to kernel buses easier. Please let us know of any issues.