The following is general advice on using multiple audio interfaces. It is not a recommendation on specific gear but I use the pieces of equipment that I do have as examples.
This post started out as a response to a post on Facebook. It has been edited a little for this particular social media.
If you are using a Yamaha MG12XU audio outs to the Focusrite, it should be easier. Then you have to get the settings right on the Focusrite.
If you are using both the MG12XU and Focusrite as an aggregate audio interface, you will have to select the proper audio interface for an output in your DAW.
Focusrite also has its own user interface. In the past, when using a Focusrite 6i6 and a Behringer UMC404HD, I had to select the 6i6 for default output in the aggregate audio interface and in your DAW. If you do this and are still not getting sound then open the user interface for the Focusrite. A lot of the time, you can solve your problem by selecting the default settings on the Focusrite. If that does not work, you might have to do some reading of the user manual.
However, you might want to take back a couple of steps. Simplifying your setup usually makes things easier to get it running. I suggest only connecting your Focusrite and get that running and sound output from it.
Then make your aggregate audio interface and work to get that running.
One more thing: if your computer is a PC, you might have a more difficult time setting up an aggregate audio interface. In general, I have had trouble doing that with a Windows PC. If using a PC and an aggregate audio interface is not working for you, consider taking the MG12XU mains out into the audio ins of your Focusrite and work with it that way.
Macs are generally much easier when it comes to setting up an aggregate audio interface.
Take the time you look up tutorials on YouTube. They will help you a lot. And reading user manuals can also help a lot although it can add some time to your work. But the time is often worth it.
