First, both your S1 Pro systems must be running the latest firmware. You can get that using a PC or a Mac. Visit http://btu.bose.com
Please watch the video below up to 1:30 seconds. It's all equally applicable to the S1 Pro systems except the pictures show the Bose Soundlink Revolve. Note: after 1:30 - the video talks about connecting without the Bose Connect app. This isn't supported on the S1 Pro system.