It's impossible to save both because if you save both, it means you're throwing both away by doing nothing because there's only one injection and you need to think longer which both will die. If you're stuck in a situation of two people are in critical condition (death's door) and you're the only one that can save them then you need to use the other one as a stepping stone in order for the other one to live. Don't take my comment as an insult. I don't want Erwin and Armin to both die nor I don't one of them to die just that a character from a different anime taught me that it's impossible to save both when you're the only one that can save them especially your enemies are stronger than you.
Im thinking abt that too like divide the serum, and inject to the both of them, but i feel like they'll turn ito abnormal titans if the serum is not fully injected to one of them haha wat a shitty shit idea *hoping abt it anyway*
if you have every seen Hunter x Hunter (2011), when Gon, Kurapika, and Leorio where asked the question by the old lady, "your son and daughter are in harm, and you were given one choice, who would you choose to rescue?" although in their case the answer was silence, Gon kept pondering and even commented about who would he have to choose if it involved someone he loved. its truly a tough situation and choice.

I only have one question tho...
IS IT POSSIBLE TO SAVE BOTH LIKE PLEASE SOMEONE? SENSEI JUST TURNED THE PLOT INTO THE CRUEL ONE ONCE AGAIN T__T