You could try using the Parallel activity with 2 Invoke Workflows. If that doesn’t work, then you would need to create something in a different language outside of UiPath like vbscript, powershell, or something else which would let you run multiple scripts at the same time and you can call them from within UiPath.
It is true that you can only run robot at once on the same desktop (although they can run on different desktops, same machine if you use Windows Server OS or there is a workaround that let’s you do it in Windows 7/10 -not recommended.) So you can definitely run mulitple robots on the same machine as long as it’s a different User ID associated with it. If you want to run them on the same User ID at the same time, then you would need to take the approaches I mentioned of using Parallel or collaborating other scripting languages to run in the background.
That’s from my understanding anyway.