Are you launching rtst as admin also? It should still work even starting it after ps2... Just not recommended on 8.1 x64.
You can also uncheck and recheck enable direct x with testing it, that's the same thing as restarting rtst.
If both are running you may want to triggering a device reset, by switching ps2 res / going full screen to windowed or back. Rtst may pick up on that and start working.