Audio Connection in Wavelab Cast -won't recognise different rec/playback devices

I’m using a MacBook Pro and have an external microphone connected via a Focusrite 2i2. I’d like to set playback through the laptop speakers and input through the Focusrite. Wavelab Cast however changes the audio device on the Rec tab to whatever I select on the Playback tab and vice versa, which is very frustrating. I’m using headphones for now with recording and playback via the Focusrite but I’d really like the option to work as described above.

WaveLab can only access one device. But on the Mac, you can create an aggregate device (Combine multiple audio interfaces by creating an Aggregate Device - Apple Support)

Thank you. I would never have worked that out on my own!