MySQLSession mysql_session;
Sqlite3Session sqlite3_session;
... login etc...

SQL = mysql_session; // MySQL will be default

Sql mysql; // default is MySQL, no need to specify session
Sql sqlite3(sqlite3_session); // explicitly specify that this is connected to sqlite3

mysql * Select(FOO).From(BAR);
while(mysql.Fetch())
    sqlite3 * Insert(BAR)(FOO, mysql[0]);