Without hosting the file on FileMaker Server you will never be certain which file, or machine, is the "boss".regardless whether you use a cloud site or simply swap files from machine to machine you'll never really know if one file is in effect over writing data that is new in the other file. There are sync solutions out there though and you may want to do a search for them.
There are third party solutions that synchronize databases. Some of them may need a server. There are several methods to synk databases without a server - not an easy task, though..
- use of UUIDs
- create a file, a list, whatever, containing Your synk data ('payload')
- transfer the payload to the other machine, using url-scripting, http(post), email, airdrop, cloud, whatever
- start a predefined process on the target machine
- loop through the payload, determine what should be done with each 'record' of the payload data
one-way syncs are easier, btw.