Late 90's / early 00's animated film involving psychic soldiers/guinea pigs

Along the years, I’ve managed to find nearly every anime I caught glimpses of on analog television as a kid. But this particular one has left me puzzled for well over a decade.

What I can remember is vague, yet distinct:

• At one point there is some sort of standoff between 2 psychically enhanced (former) soldiers;

• There are flashbacks of war (fought in the jungle?) amidst mental battling and blood puking set to a sparse background (mainly white);

• I vividly remember one of the guys having this ‘double neck’, similar to the Pokémon Mewtwo.

Another thing to note is that I’m sure I watched this on the Dutch version of Canal+ in the (very) early 2000’s.

As I haven’t been able to find this ‘anime’ after so many years, I’ve begun to wonder whether this even was a Japanese animation, or if it was an animated short-film from God knows where. The latter would explain my struggle, but it also makes the continuing search that much harder.