I have two 7770's in CrossFire, but for the life of me I can't get GUIMiner to use both of them. In the "device" drop down menu, it shows both of them, but how do I get them to run at the same time?
You can create a new "miner" from the File menu, or duplicate your existing miner in GUIMiner by right-clicking the tab and clicking Duplicate. Setup one miner for one card, and another for the other. Then click View>Show summary and you can start/stop the miners from there.
Here's a
picture of the final setup in case it helps. I'm also using the OS X version of GUIMiner so hopefully the steps are the same for you.