I feel like I was thrown back into the old days of nintendo with gimmicky gameplay and twists. I love the idea of controlling two characters at once and also love the open interpretation of what's happening.
I say these two characters are soul-bound in the mirror worlds. The blue guys can't climb unless there's another who entered in the real world. The pile of dead bodies also means that this is a repeated cycle. I'm going to assume that the blue bottle is a type of elixir of life or something that is either good or disastrous if obtained. If this is a repeated cycle, the first blue guy we control suffered the same fate as previous, but remains blinded by greed. It's those thoughts of "if I can't have it, then no one can," thus preventing the guy in the real world from getting this "drink" and pulling him into the mirror world after killing itself. When the guy from the real world was dragged and saw the mountain of bodies, it made a conscious decision to let the new guy get the bottle at the end.
Though, why did everything turned gray at the end? It could mean, both characters were pessimistic. The new guy had apprehension but was surprised he can jump while the old guy probably knows he'll be stuck in the mirror world forever. That's my interpretation.
Looking forward to the next project you'll make.