Welcome to Apple Support Communities.
Not trying to be redundant about things you've already tried, but you didn't mention it, so I'll offer this:
Have you tried a PRAM after installing both 4GB modules?
I had a similar problem with a RAM upgrade in a 2009 MacBook Pro.
Once PRAM was reset at power-on (hold down Command-Option-P-R and wait for it to reboot twice before releasing the keys) it worked.
An SMC reset wouldn't seem to be related to RAM, but it doesn't hurt anything to try: