After a few tries, I still can't make this work, it seems that I'm missing something trivial.
Here is what I wrote in textures lump:
texture HPROCKXX, 256, 256
{
patch HPROCK1A, 0, 128
patch HPROCK1B, 0, 0
}
HPROCK1A and B are graphic files that should be both part of the texture (HPROCKXX).
Should I make some sort of additional lump for the texture itself, or maybe there are some markers missing?