What happens when you have two connected network interfaces? I have two connected network interfaces, ethernet and WLAN.  How does OSX handle traffic in this situation? When outgoing messages are sent which interface are they sent from? If I use tools such as Logmein.com, which interface does it connect through?
It depends on the service order set in System Preferences->Network. The first one that has a route to the requested endpoint will be used.

Click on the wheel icon at the bottom left and choose Set Service Order.... Then drag the items into the required order.

This is the order on m machine which I do not think I have altered.