Try to use DataSource property for your names combobox.
kmt = new System.Data.OleDb.OleDbCommand("SELECT ProductID, Productname FROM Products");
kmt.Connection = bag;
tablo = new DataTable("Products");
System.Data.OleDb.OleDbDataAdapter oda = new System.Data.OleDb.OleDbDataAdapter(kmt);
oda.Fill(tablo);
comboBox1.DataSource = tablo;
comboBox1.ValueMember = "ProductID";
comboBox1.DisplayMember = "ProductName";
Then add to your comboBox1_SelectedIndexChanged method the following:
if( !System.Convert.IsDBNull(comboBox1.SelectedValue) ){
System.Data.OleDb.OleDbCommand cmd = new System.Data.OleDb.OleDbCommand();
string query = "SELECT id, ProductID, Model FROM ProductData\n" +
"WHERE ProductID = @pid";
cmd.Parameters.AddWithValue( "@pid", comboBox1.SelectedValue );
cmd.CommandType = CommandType.Text
cmd.CommandText = query;
cmd..Connection = bag;
DataTable tbl2 = new DataTable("ProductData");
System.Data.OleDb.OleDbDataAdapter od = new System.Data.OleDb.OleDbDataAdapter(cmd);
od.Fill(tbl2);
comboBox2.DataSource = tbl2;
comboBox2.ValueMember = "id";
comboBox2.DisplayMember = "Model";
}