The rules are not an absolute fact. They are not there to say "robots will never harm a human, and will always save the day, when present". We know this by how the movie plays out. The rules are simply the rules used to govern the actions of the robots, in-verse. As a programmer, this is something that is blatantly obvious to me; but I am confidentconfidant that it is not as so for others that are not familiar with how strictly adherent any computer system is.
The point is, the rule does not state anything about it "not counting" because the robot is "already saving someone". As such, only considering this explicit rule (as any computer or robot would interpret, at least), there is no allowance for a situation where the robot can only save one of two people in danger. In actual computer science, only considering the explicit rule, such an event would likely cause an infinite loop. The robot would stop where it was, and continue to process the catch-22 forever; or at leastatleast, until its logic kicked it out of the thought process. At this point, the robot would dump its memory of the current event, and move on. In theory.
In verse, the rules are a lot more complicated; at least, internal to the robot. There would likely be a whole lot of special cases, when processing the core rules, to determine how to act in such situations. As a result, the robot is still able to act, and takes the most logical outcome. It only saves Will, but it does save someone.
In regards to how the robot would actually process these rules, it would not be breaking the rules, at all. There would be a far more complex series of "if..." and "else..." logic, where the robot'srobots logic
would would allow it to go against these base rules in situations where logic dictates that no matter what option, a human would still come to harm.