Dear ZP18,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, it seems that you are having concern with Data Visualizer template using Cross-Functional Process Flow Char with Excel file in Visio, need to create a process between 2 swim-lanes that creates a new function swim-lane "A+B" when updating Process 1 that is shared between function A and B in Excel table.
If the understanding above is right, we've done hard searches and test on our side, however, we have not found an out-of-box way to achieve this currently, sorry for the inconvenience caused.
Given this, we'd recommend you share feedback about the feature via Help > Feedback within Visio. Feedback submit by this way directly goes to the related team, this is the best way to let Visio developers know your requirements and comments, help improve the product and make it better for you and others.
For your reference: How do I give feedback on Microsoft 365? - Microsoft Support
At the same time, we will also keep this thread open, so other Community members and Experts can also share their suggestions and inputs.
Sorry for the inconvenience again. Your patience and understanding are highly appreciated. Hope you stay safe and healthy.
Sincerely,
Rhoda | Microsoft Community Moderator