! Attention chapter 5 fix it writers !
(Disclaimer im obviously no toxicology expert and got these infos from like 3 google searches so there may be a lot of bullshit)
I think it's well known by now that the poison in chapter is based on strychnine. The english localization directly references it, unlike the japanese version where it's simply named 'Lethal torture poison' but the symptoms are apparently similar enough that several people had already drawn the connection before the english was out.
An interesting fact about strychnine is that it's a lot more efficient, and lethal at way smaller doses, when injected intraveinously rather than ingested orally.
(In real life, strychnine doesnt have an antidote, the treatment consists of removing the toxin from the body and using muscle relaxants to stop the convulsions and prevent asphyxiation from paralysis of the respiratory muscles.)
But since it does have an antidote in the game, maybe we can suppose that it functions in a similar way. (I have no idea if that's the case for actual antidotes, are they build molecularly in the opposite of the poison ?) That would explain why you'd need to drink the whole bottle after being poisoned cutaneously from the arrow, since drinking it would lessen the efficiency.
So by this logic, if they were to inject the antidote instead of drink it, there is a possibility that there would be enough to save both of them.
So in conclusion, if you want a not-too-complex way for both Momota and Ouma to survive chapter 5, a simple syringe could do the trick.