Yes, built-in Screen Sharing will work across multiple ISP's as long as the two Macs can see one another. VNC in particular is a robust broadly support albeit it not particularly secure protocol. The routers on each end (not the ISP) but the local routers have to be set to allow at least VNC connections (illustrated guides for most routers).
To get across wide internet gaps and deal with automated login, there used to be a BackToMyMac feature in MobileMe which worked well and has now moved to iCloud and is free for users of iCloud (not me).
You can also access screen sharing via the "Connect To Server" (command + k) from the finder. Once there put in vnc:// followed by the host name or ip address.
Chicken of the VNC (now just Chicken) is a great free VNC client. JollyFastVNC is an even better (although paid) client (licenses $10 or $30).
There are several competing Mac compatible solutions. With these solutions, you don't have to worry about router configuration as the connection handshake goes through a central server and they remap the traffic to common http protocols, open on every internet connected network.
- Remotix will allow you to use VNC or Apple Remote Desktop with computers located anyway. Very good reviews at MacUpdate from pros. $40.
- TeamViewer will allow you to control remote screens cross-platform. Personal use is free. Commercial use is very expensive. I've used TeamViewer quite a bit in the past and it's good but we had to give it up at work. New versions with stiff upgrades seem to come every year. TeamViewer is on v11 now and was v3/4 when I was using it.
- LogMeIn is paid use only and has angered their users. I've not used it.
Remote screen sharing is viable but it is significant IT overhead and does entail substantial security risks. If you can log on, so can a good hacker. There are automated bots out there scanning for open ports and machines which answer.