1) You cannot set an override to run early in CA Workload Automation AE (Autosys).
Once the one-time override is run and completed, it will schedule itself for its next available run time.
So if it completes before the NORMALLY scheduled time, the job would be scheduled again for that same day/time as normal. If the override job ran such that it did not complete until AFTER its normally scheduled run in that instance, it would schedule itself for the NEXT available future time... so the next day assuming the job is scheduled to run once a day.
2) If you place 2 One-Time overrides, the first override is deleted.
You can only have one override at a time, until it is executed.