If you want to cook steaks at different doneness, it’s best to cook the medium-well first. When it’s done, reduce the heat for the medium steak and leave the medium-well in the bath to keep warm. You might find these threads/topics in the community helpful:
- Cooking for two
- Is it possible to cook 2 steaks with different doneness in the same batch?
- Different doneness
Temps for beef tenderloin, you can try 140F/60C for medium and 145F/62.7C for medium-well. 