Stereo grouping is limited to Apple home and Airplay 2. Roon cannot use airplay 2 to send as airplay2 is limited to iOS or Mac devices system airplay which it doesn’t use. It uses its own revered engineered airplay 1 so all non Mac users get same function. Airplay by default is limited to Apple products.
It is do-able. However, it is a workaround and a little long-winded. If you want, I’ll post instructions on how to do (should you wish to tinker). It’s how I used mine. You’ll have to use Roons DSP Procedural EQ and mixer to do the stereo split. Once done, you can group them together.
Actually, I found the original post which will save me having to write it!
Alternately, if you use Macs or iOS devices such as iPhones or iPads, you can play to the system output of these devices if they are set to play to the paired HomePods. I do this with my HomePod minis when I connect to them via AirPlay to my MacBook Pro or iPhone.