“The trial version allows me to choose only one table at the time” no .. TWO!
“The result is that the source db overwrites the target db” This is by design!.
Please search a little! This has been asked 100 times and replied to 100 times as well!
Short answer: there is NO WAY a computer can tell which row is the correct one if two rows with the same PK_value exists on both hosts, it needs a 'rule' defined by a human. The only 'rule' supported at a time is “overwrites ” We plan a more advanced 'conflict resolver' for users to define their own rules.