If you've checked your hardware and everything's good, your setup is wired properly, and you still can't enable SLI due to a consistent Nvidia driver crash or monitors that permanently "black out" until you restart your system, please ensure your copy of Windows is legit.
Recently, cracked copies of Windows have been preventing users from enabling SLI after the users enable the "Windows crack" file. After the users downloaded and installed legit copies of Windows, users were able to enable SLI without hassle.
For some reason, illegitimate copies of Windows do not seem to be affecting the enabling of Crossfire. Only SLI is affected by this.