I kinda think you got infected by both- even though it wasn't really implied early game. Both monsters were there and chatting around your body- even though the initial scene where you get the stabby stabby juice is either one or the other monster. The MC was very unconscious- so pretty much anything could have happened. It wouldn't make a ton of sense the other way since the monsters in the game seem to exhibit different abilities baseline- and either route the MC has the same trait (or traits?). Your girl Ella kinda implies it as well when she gets all happy you have "both mutations" later in the game. Cannot remember which of two events she decides to kill you in it is said in. I think it was when you revealed how you can mini-tentacle to her - when she asks about the extent of your abilities.