Is there any way to avoid item to mix when entering an underground beltlayer tunnel?, I tried to increase the buffer size (wich is what instructions tell you to do if you use more than one item per belt), but nothing, still mixing

I don't know if I made myself clear, i use belts to carry one item per side, trying not to have more than that in each side, and beltlayer doesn't respect that sistem

As the documentation says, this is a limitation of Factorio’s loader entities.

Meaning... Nothing can be done?

Correct. I may look into making Miniloader be able to produce two-sided belts, which might work for your purposes, but the description of Beltlayer is quite clear:

Items are mixed randomly in the left and right lanes of output belts. This makes Beltlayer only usable with belts carrying the same item in both lanes, or "sushi belt" designs that don't have any item placement requirements.