Is it possible to sync two servers separated by NAT?

Server
1

I've setup an ownCloud server behind a NAT network. Now I want to access it outside the NAT, but I don't have access to the router so I cannot simply setup a port forwarding to solve the problem. So I want to setup another ownCloud server outside the NAT, which will have a public IP. Now I'm wondering whether it is possible to sync files between them.

The reason why I don't simply only use the public ownCloud server is because uploading and downloading files from LAN server is much faster.

2

You can use federated sharing between them, but the data will stay at the original server. Other option is two sync clients with a file system in between as intermediary ...

3

But you can only reach the public server from the server behind the NAT. Not the other way round. Did you consider using a VPN? So both servers can talk to each other via VPN. You can even use a VPN service to get you server behind NAT a public IP.