col1 = 5 'Premiere colonne
col2 = 7 'Seconde colonne
r1 = Cells(2, col1).Address(ColumnAbsolute:=False, rowAbsolute:=False)
r2 = Cells(2, col2).Address(ColumnAbsolute:=False, rowAbsolute:=False)
'....
.Cells(2, "AT").FormulaLocal = "=" & r1 & "&" & r2