OpenBox lacks support for _NET_WM_FULLSCREEN_MONITORS. This was discussed on bug 5182, but that bug was about problems on Thinstation and is now closed.
The lack of support for _NET_WM_FULLSCREEN_MONITORS means that you cannot run any applications with tl-single-app which wants to go fullscreen over multiple monitors. This could be vncviewer/tlclient, x11clone, FreeRDP or some other application.
Upstream bug: https://bugzilla.icculus.org/show_bug.cgi?id=6246
(In reply to comment #0)
> Upstream bug: https://bugzilla.icculus.org/show_bug.cgi?id=6246
More than 4 years has passed, but unfortunately no progress.
According to my experience with common WMs, "xfwm4" is the only WM that works correctly. Perhaps we should consider shipping that one instead?