That's true, but it will still deliver a much better result than cooking non-sous vide. And you're talking maybe a 2-4 minute extra cook time for the medium steak, since you're most of the way there, so you're not going to get that much of a gradient. (And it's not like the universe comes to a screeching halt if one of the steaks has a slight gradient to it, especially for someone who prefers medium steaks.)
On the other hand, cooking one steak at medium in the sous vide, letting the temp drop and then cooking the other at rare, will add at least another hour (maybe two, depending on your desired sous vide time) to your overall prep time. That method will certainly give you a better result, but that's a lot of extra time for a two-person steak meal.
As with many things in the kitchen, there's an "ideal" method and there's the almost-as-good-but-much-quicker shortcut method. Cook's are free to choose.