Please note that this tutorial was written for Logic Pro 9 and although the basic principles are likely to remain the same for future updates, some of the screenshots may not exactly match up with what you see in future versions.
To use an aggregate device within Logic Pro:
1. Firstly you will need to setup an aggregate device on your Mac. If you haven't already done so, Click Here to learn how to do this.
2. Once you have created your aggregate device, open Logic Pro (or restart if already open).
3. Navigate to Logic Pro → Preferences → Audio in the Logic menu bar.
4. Select the ‘Devices’ tab, followed by the ‘Core Audio’ sub-tab.
5. Select the name of your Aggregate Device from the relevant ‘Output Device:’ and/or ‘Input Device:’ drop down menu(s).
6. Select ‘Apply Changes’.
Your aggregate device should now be set up for use with Logic Pro, i.e. you should now be able to record into Logic using your combination of interfaces (aggregate device) and all of Logic's sound output should also pass through the specified interface(s) making up the aggregate device (e.g. out of any speakers connected to the outputs of the interface(s)).