Unattended flows by definition run in their own user context in a user session. So unless the target machine where you want to run the flow is a server based OS (one that allows multiple concurrent logins) then you won't be able to run unattended in the background while another user runs in the foreground. There is no foreground/background in Desktop flows. Everything runs on the desktop. Unfortunately, the scenario you describe isn't feasible with either attended or unattended desktop flows.
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Paul Papanek Stork, MVP
Blog: https://www.dontpapanic.com/blog