Hey Skyler, I'm having the same issue. It appears that if you have at least one DIMM of 1066MHz RAM, the machine will be able to boot and clock-down faster RAM so that both DIMMs are running at this MacBook's maximum 1066MHz speed. I suspect that because both of my upgraded DIMMs are faster than 1066MHz (1300MHz), it is preventing the machine from being able to boot, and it's subsequently unable to remedy the RAM speed discrepancy. In my case, I get a single beep every 5 seconds when trying to boot which means "No RAM Installed"

The solution would be to return the faster memory and buy 1066MHz speed RAM, although now-a-days that's more expensive than buying 1300MHz or 1600MHz RAM when you're looking for 16GB. I guess you can also leave the mismatch DIMMs (10GB) if you need to, but it's definitely not optimal.