Re: PHP/Mysql Multiple Questions
1) I don't have a 100% positive answer but I don't think this is possible since each SQL statement is executed in the context of the opened db connection pointer. Sure you can have multiple pointers open to different DB servers but I don't believe there is a way to join these two pointers in an INNER JOIN/LEFT JOIN style syntax. I believe you'd have to do recordset enumeration in PHP and possibly some additional nested queries.
2) How many records? I guess you could use two arrays and compare them, inserting objects not found in array1 to db2 and array2 to db1.