My husband and I both downloaded the app on our phones. We were at a show and wanted to relieve one while the other took a break. The app would not allow both of us to be signed on at the same time on different phones. 
After the show, I experimented a bit, found out the set up hadn’t been quite completed, and uninstalled and reinstalled the app on both phones. That seemed to clear up the issue.  Being new to square, first time use was a bit frustrating. I believe I have it corrected. 

Ah.  Well I’m glad to know that you got it resolved so you could go about your actual business.  I know that new apps/systems can be frustrating at times, but it sounds like you had successful troubleshooting.  If you need anything else, there are lots of fellow Square sellers here, and we are ready to help.

Best wishes,