$Acuit = Array();
$stmt = $mysqliSOL->prepare("SELECT cuit,RAZON_SOCIAL FROM cobranzas_padron
                                GROUP BY CUIT,RAZON_SOCIAL
                            UNION ALL
                            SELECT CUIT,RAZON_SOCIAL FROM cobranzas
                                GROUP BY CUIT,RAZON_SOCIAL

                            ORDER BY RAZON_SOCIAL ASC");
$stmt->execute();
$stmt->bind_result($cuit,$razon_social);
while ($stmt->fetch()){
    $Acuit[$cuit] = $razon_social;
    //$Acod[] = Array($cuit, $razon_social);
    //$AcodDesc[$cuit] = $razon_social;
}
$stmt->close();

foreach ($Acuit as $cuit=>$razon_social){
    $Acod[] = Array($cuit, $razon_social);
    //$AcodDesc[$cuit] = $razon_social;
}