QSqlRelationalTableModel not working when having two relations to same table

I use QSqlRelationalTableModel to show data. It does not show any data if I uncomment

// mpCustomerModel->setRelation(18, QSqlRelation("CountryMaster", "code", "name")); 

in the below code. Is there something wrong?

mpCustomerModel = new QSqlRelationalTableModel(this);
mpCountryModel  = new QSqlTableModel(this);

if (NULL != mpCustomerModel)
{
    mpCustomerModel->setTable("CustomerMaster");
    mpCustomerModel->setRelation(15, QSqlRelation("CountryMaster", "code", "name"));
    //mpCustomerModel->setRelation(18, QSqlRelation("CountryMaster", "code", "name"));
    mpCustomerModel->select();

    ui->customerComboBox->setModel(mpCustomerModel);
    ui->customerComboBox->setModelColumn(1);