I've been using Drop box and opening files from the drop box folder with no problem. The problem lies in making sure that two users do not both open the file from drop box at the same time. That will be a problem, not sure if you'll end up with a damaged file or if the last user to close the file will overwrite the other users' edits, but either won't be good. (And the second user might just be locked out of opening the file, but this is much like accessing a file on a shared directory and the results endanger the file's integrity--so it's not a good idea however you slice it.
I think you'll need to consider either a file syncing strategy or paying to have a third party host the file for you.
There are shared Filemaker host web sites online where for $20 per month or less you can place one shared file that the two of you can open, even simultaneously.
If you do this you must create accounts and passwords to keep the entire world out of your file. Your buddies sharing that host can try to open your file and if it is not protected they can get in and whatever. The entire world can try to open your file if they should get the fmnet address whcih you will probably send to your partner via unprotected text email rather than using (read a W.E.B. Griffin novel) an encrypted secure sat telephone call. :)
Think of a shared host site as the 'original cloud'.
The next try is to locate a freind who already has a Filemaker Server with Internet access and convince him/her that it is a good idea to host your file for you...
(They don't need FM Server - if you buy a Nice Friend a copy of FM Client they can host the file for you.)
Thanks for the advise. I will look into the Hosting option.
I guess a common Drop box account could be another solution.
I do not recommend a common drop box folder (doesn't have to be same account). It will appear to work, but please note the potential problems that can occur with such a set up that I listed in my first post.