Anita, your last question first. Yes, you can leave both steaks in the hot water, - for a reasonable length of time, until time to sear. Leave the water temperature at your desired medium rare cooking temperature.
As Alyssa explained, you first cook the medium steak. It will not cook any more at the lower medium rare temperature. Water temperature regulates doneness. Please trust us.
Successful sous vide cooking requires all new thinking on your part. Anita's greatest challenge will be to stop applying conventional cooking thinking to her sous vide technique. You can start by remembering the following: - more-time-does-not-mean-more-doneness. Close your eyes and repeat that three times. Better do that quietly if other people are nearby and try not to move your lips.
You are now working with new cooking rules. You already have Rule #1. If the medium rare steak is cold the water temperature might drop to your desired and hopefully already reset medium rare doneness temperature. Otherwise have a handful of ice cubes ready. You probably only want a 5F drop in temperature. Then having already calculated your medium steak's required length of time to cook by using its thickness as your guide you are all set.
Don't forget to pat dry and apply ample Kosher salt and freshly ground black pepper before searing. Go to it and enjoy your two great steaks.