Hi Telecaster33 and welcome to the Bose Pro-Portable Community!
To my understanding you are unable to Party/Stereo Mode the S1 Pro Speakers within the Bose Connect App. If this is correct, I recommend updating your S1 Pro Systems to the latest firmware via the btu.bose.com updater tool. You will need to connect your S1 Pro's 1 at a time to your computer using a Micro USB Cable and once the updater tool has been installed, your web browser should automatically refresh and acknowledge that your S1 Pro is connected. At this stage follow, the instructions on screen to proceed with the manual update.
Once both S1 Pro Systems have been successfully updated, try again to pair them in Party or Stereo Mode within the Bose Connect App.