On my MacBook Pro, High Sierra 10.13.6, Airplay 2 now enables me to play iTunes to both of my Homepods.
Using JRiver MediaCenter 24 I can only play to one or the other of the Homepods in the room.
Airfoil, which was helpful before, does 'see' the combo of both Homepods but when I select it, only one of the Homepods is actually used. They might be able to sort that out eventually, I hope.
Has anyone been able yet to play JRiver MediaCenter to both Homepods? Is there a development in the pipeline to enable this?