My inefficient way to do that now is concatenate 2 fields (using a vb script) in another field "x" which has an "a:item = "x" ".
The problem with this is that the item can't get a further update if the field x is already populated.
I wanted to put an equation in the item field (=NameConcatenation(contact,item,FirstName,LastName)
and have a corresponding VB function like :
Function NameConcatenation(contact,item,FirstName,LastName) ' Concaténation des prénoms et noms
if item = "" and contact = -1 then
NameConcatenation = FirstName & " " & LastName
elseif item <> "" and contact = -1 and (FirstName <> "" and LastName <> "") then