Took me a little while to get this sorted out so I thought I’d document it here:

Music Assistant supports grouping speakers so, briefly put what you’re doing here is just creating a group with two HomePods, one playing the left channel, the other the right channel. Key here is configuring the individual HomePods to play left/right and then creating the group.

Details:

Under Music Assistant Settings, select the “players” icon (in this screenshot it has the red dot on it)

Select one of your HomePods and expand the “Audio” settings section. At the bottom you can select the channel:

Pick either the left or right, depending on your choice/location of the HomePod. Don’t forget to hit “Save” and then repeat this process for the other member of the stereo pair.

Once you’ve defined your HomePods as left/right, go back to the settings and select “Add Group Player”

From there it’s just a matter of select “Group Type” of “AirPlay” from the drop down and adding your two HomePods the group and give it a useful name like “Stereo Pair” this seems to be pretty reliably working though sometimes I have to restart the stream once or twice