I found a solution to my problem:

Select Client
If Empty(Tag(1))
Set index to Cnum.cdx
endif
Set order to tag Cnum

I hope this won't affect anything else...

>>No they are two differents sessions opened in the same form (Open twice).
>Yes Benoit you're right. Unfortunately I can't think of a workaround other than a view right now.
>Cetin