Set up two HomePods in a stereo configuration, and they act as one pair when used via Siri or an iOS device. Open the HBO, YouTube, or Music app on your phone, and voilà, stereo sound.
This doesn’t seem to be an option on a Mac. On my 2017 MacBook Pro, I can only choose one HomePod at a time as an audio out. I’m running macOS High Sierra 10.13.6. [Update: It’s the same on macOS 10.14 Mojave and 10.15 Catalina… Here's hoping for Big Sur.]
Is there a way to make this work?