Screen sharing between Macs is very easy as long as they are located in the same local network.
Once the are in different networks it gets a little more – no not complicated, it’s just not that obvious.
“You can, however, connect to computers on other network segments if you know their IP addresses.
To do this you’ll need to launch the Screen Sharing application manually. The application is located inside the Core Services folder along with several other Mac OS X components that are normally launched as part of operating system (including the Finder).
This folder is located at the following path (beginning at the root level of your hard drive): /System/Library/Core Services.”