This is done just by separating the different queries with a semicolon:
public DataTable GetData2()
{
Npgsql.NpgsqlConnection oConn = new Npgsql.NpgsqlConnection("Server=192.168.10.4;Port=5432;Userid=postgres;Password=stefan;Protocol=3;SSL=false;Pooling=true;MinPoolSize=1;MaxPoolSize=20;Encoding=UNICODE;Timeout=15;SslMode=Disable;Database=pagila");
oConn.Open();
DataSet oDataSet = new System.Data.DataSet();
Npgsql.NpgsqlCommand command = new Npgsql.NpgsqlCommand("select * from city; select * from country", oConn);
// Now add the parameter to the parameter collection of the command specifying its type.
// command.Parameters.Add(new Npgsql.NpgsqlParameter("col1", DbType.Int32));
// Now, add a value to it and later execute the command as usual.
// command.Parameters[0].Value = 8;
Npgsql.NpgsqlDataAdapter oAdapter = new Npgsql.NpgsqlDataAdapter(command);
oAdapter.Fill(oDataSet);
DataTable dt = oDataSet.Tables[0];
DataTable dt2 = oDataSet.Tables[1];
oConn.Close();
return dt2;
}