dataSet = new DataSet();
adMarks = new SqlDataAdapter("SELECT * FROM marks", connection);
adForms = new SqlDataAdapter("SELECT * FROM forms", connection);

adMarks.Fill(dataSet, "Marks");
adForms.Fill(dataSet, "Forms");
            
cbSelectMarka.DataSource = dataSet.Tables["Marks"];
cbSelectMarka.DisplayMember = "name";

cbSelectForma.DataSource = dataSet.Tables["Forms"];
cbSelectForma.DisplayMember = "name";

DataRowView row = (DataRowView) cbSelectMarka.SelectedValue;
SqlCommand cmd = new SqlCommand("select forms.name from forms,MarkaForma where MarkaForma.id_marka = @id and MarkaForma.id_forma=forms.id",connection);
cmd.Parameters.Add("@id", SqlDbType.Int);
cmd.Parameters["@id"].Value = row["id"];

adForms.SelectCommand = cmd;
adForms.Fill(dataSet,"SelMar");
cbSelectForma.DataSource = null;
cbSelectForma.DataSource = dataSet.Tables["SelMar"];
cbSelectForma.DisplayMember = "name";
cbSelectForma.Update();