In chapter 84 of Attack on Titan,
Levi decided to inject Armin with the Titan serum to have transformed the almost dead Armin into a Titan so he can consume Bertholdt to gain his Colossal Titan's abilities.
Why doesn't Levi just inject both Armin and Erwin with half of the serum each so both survive?
I can't remember if anyone, Levi included, actually saw when he tasted the serum, did they? And either way, there was one guarantee to get one of them into action, or a chance to get both, but if it didn't work for any reason, everyone would get ripped apart or eaten alive. I watched the show, so could have missed some info from the manga, but sounds like good reasoning to me.
A question that we should also think upon: Would Erwin have wanted Levi to turn him into a mindless Titan just so he could 'live'?
Remember that the Titan injection turned Armin into a Normal Titan, not a Titan shifter. He still needed to eat Bertolt afterwards. If both turned into Normal Titans, only one would still be able to inherit the Colossal Titan. There has been no precedence in the manga nor a mention of an event where two people inherited the same Titan powers but a person may acquire multiple Titan shifter abilities.
Besides, as seen in Chapter 84 and to quote from the wiki, Levi himself did not want to bring back Erwin, even if there was a way:
Levi notes that Erwin became a demon just as they had wished, and that he deserves to rest, rather than be brought back into their hellish world.
So, no, only one would still survive.
Only a little bit of the serum is needed to transform an Eldian into a pure titan. To become a titan shifter, however, a pure titan needs to ingest the spinal fluid of a person who is already a titan shifter.
That means that both Erwin and Armin could have survived. All they had to do was inject both of them with the serum and feed them each half of Bertholdt. The only problem is the possibility that there is not enough of Bertholdt's spinal fluid to go around for Erwin and Armin to both transforms into titan shifters and gain the power of the colossal titan.
They only had one titan shifter that is Berthold. Even if Levi injected both of them, one of them had to be a mindless titan. If they had successfully captured both Berthold and Reiner, both of them would have been injected.
