What's the best way to prepare two steaks via sous vide if you want to cook a medium rare and a medium steak? Is it best to cook the medium steak first at its temperature, then lower the temperature, leave the medium steak in and cook the medium rare steak?
Pretty simple, cook the two steaks separately at the required temperatures the day before. Cool in an ice bath and reserve in the fridge. The next day rethermalise the steaks in the bath at say 55C: you really only need to heat them for around 25-30 minutes, as all you want to do is take the chill out of the steaks before you sear them on as a high a heat as possible. Do not season your steaks before they go in the bag because the meat will cure slightly overnight in the fridge.
There's some good info on Cooking Issue around the cook/chill method, especially the pre-salting question:
There are two main types of low-temperature cooking – direct-serve and cook-chill. For direct-serve you cook foods and serve them right away. For cook-chill, foods are cooked, chilled, stored, and rethermalized at service time. Here is a typical sequence for cooking a steak:
Sear the raw rib-eye. This step kills bacteria on the surface, starts the browning reactions that contribute to good meat flavors, and ensures that a nice crust will be formed quickly later when the meat is seared again.
Put meat in a bag with butter.
Cook the steak for 1-4 hours at 55 C. The optimum length of time depends on how tough the particular steak is.
- Pull the steak out of the bag, sear it (to make a nice crust) and serve it. This is Direct Serve.
Chill the meat and store.
Retherm it at 52 C.
- Pull the steak out of the bag, sear it and serve it. This is Cook-Chill.
Cook the medium steak at the temperature you desire until its done. The add ice or cold water to lower the temp for the rare steak. The first steak will only stay warm, but not cook past medium since you have lowered the temp. When the second steak is fully cooked then remove and char both.