Hello -- I have an Asus Notebook with HP dual monitor using NVIDIA GeForce 9300GS. I have not had a problem for over a year and just recently started having this problem. I can remember it happening after removing the laptop from the usb ports to walk away (wireless).
When I manually sleep/shutdown and/or restart the laptop, the HP screen flashes as if it were to wake-up then it "monitor going to sleep" and only the laptop displays requiring me to "set up multiple displays".
If I allow the system to go to sleep on it's own, upon waking up/moving mouse both laptop and dual screen wake up without any problems.
I've run driver diagnostics etc. and everything is working fine.