How to force a template for an item in many categories ?

Sorry I mix myself.
What we want is : when we display a category, we need that the template of this category displays the items.
And when we display another category (with, sometimes, items published in both categories), we need that the template of this other category displays the items.
I'll come back with examples.

Hello

i think that i understand you

the category A uses (the category layout) of 'template' A
the category B uses (the category layout) of 'template' B

item D is assigned to both categories A and B

you would want the item D to be shown using
- the (item layout) of template A when click via link in category view A
- the (item layout) of template B when click via link in category view B

Hello

it is possible and simple to do it
without any change in flexicontent core package

-1- in the item type you can select the "allowed" layouts (or leave it empty meaning all layouts are allowed)
-2- then in category_items(_html).php of yout custom template, at the line that creates the item links add:

Of course we could add a new parameter for this category configuration so that our templates can do this out of the box
but i think this usage case is rare ?