You cannot build a stereo pair, similar to the speakers, where both PORT’s become a single Room, but you could Group the PORTS and connect only one channel to each PORT.

Thanks, @buzz.

What is the format of the audio that comes out of the digital output? Is it stereo? I can configure the speakers themselves to use left or right channels (plus center, l surround, and r surround). Then I could, as you say, use two ports as a room group rather than an actual pair and configure the speakers accordingly. Yes?

It is stereo.  You can’t use ports for a surrounds sound setup if that’s what you’re going for.  Note that if you do this setup, each port would have it’s on volume control.  You can have them play the same volume, but it won’t be automatic like a typical stereo pair.  

Note that there is a 75ms latency between Line-In and output from a PORT, or any other SONOS player. This would thwart any attempt to use this arrangement in a surround system.

Thanks, all. I don’t want to use this for surround sound. I just happen to have all of these speakers. I thought I’d get some ports and make them into glorified boomboxes haha.