It is possible, though you will need to configure the router for the host machine so that it allows and forwards traffic on port 5003 to the local network IP address of the machine hosting the FileMaker file, which will have to have the file open and network sharing turned on for it. You will also need to know your external IP address so the remote user can connect to the host - which you can easily find by going to whatismyip.com
The setup is something like shown in attached image. There are also other methods such as using VPN 'mesh' networks etc such as Hamachi LogMeIn - www.vpn.net - which means that you don't have to configure your router etc...
Yes you are able to do share between two computers not on the same network.
This is done by setting up port forwarding on router were your host database.
I have included a link bellow which covers the process
It is best the host network has a fixed IP address
Yep - that page the link goes to has good information :-)