but i would like to know the meaning of the formula you sent me :

=CHOOSE(condition1+2*condition2+1, neither, 1_only, 2_only, both)

...and in particular : 2*condition2+1

There are 4 possibilities:
1. condition1 and condition2 both FALSE
2. condition1 TRUE and condition2 FALSE
3. condition1 FALSE and condition2 TRUE
4. condition1 and condition2 both TRUE

Take a look at
condition1 + 2*condition2 + 1
Bearing in mind that FALSE=0 and TRUE=1 in this context, the four
possibilities evaluate to the numbers 1,2,3,4. This is just what's useful
as the first argument of CHOOSE.