Thanks for getting back to us, kren.
You're right that only one Square account can be connected in QuickBooks Online (QBO).
While we're unable to provide a specific app, you can visit our QuickBooks App Store. Then, find any third-party applications that are already integrated with QuickBooks Online.
For more details on how to sync Square accounts, you can check these articles.
Let me know if there are other things you need help with connecting multiple Square accounts. Just leave a comment below, and I'll get back to you.