Are you running 6.3.26? There was a problem that it would launch the second run on the same cores with that version of Fluent. If you use 6.3.35 or 12.0.16 it should not happen.
To set the cores manually:
- Start the first job to run (it will grab four cores for itself).
- start second Fluent session and load case and data
- set the process affinity with the TUI command: