They were most likely bought at separate times.
If you are that close just use ethernet, otherwise move the router at least 3ft away. Believe it or not but being too close is a bad thing.
The GTX 690 is the most powerful gpu ever made, it even outperforms the Titan and brand new Titan Black.
Geforce aren't crossfire compatible. Only radeon are, they are actually SLI which performs better than crossfire.