How can I use two network adapters on OSX, one for LAN traffic and one for Internet traffic?

Solution 1:

Mac OS X gives the default route to the highest-ranked, active interface. It doesn't have anything to do with bandwidth. To change interface rankings, go to System Preferences > Network, click the cog drop-down menu under the list of devices and select Set Service Order. Then drag to rearrange your interface rankings. It sounds like you want RED ranked above BLUE.

If BLUE is a multi-hop network (i.e. not just the local subnet), then things may get tricky. You can also add specific routes using the route command. This would allow you to route 10.0.0.0/8 addresses over "BLUE".

Solution 2:

You can confirm the routing by looking at the routing table (if you know how to interpret that):

netstat -nr -f inet

or specifically querying the routing table:

route get 204.XXX.YYY.ZZZ

for some destination, 204.XXX.YYY.ZZZ.