I use Circus Ponies Notebook. I can use it on two machines but only have 1 open at a time.
I’ve used the trial of Together and when open it writes a lock file so if you try to open the program on another machine you risk damage to the data.
I have Things and other programs which can ‘sync’ using Dropbox but if two programs are open at once it could end badly.
Someone somewhere must have written a program that basically says “Add your programs here and when the screensaver starts those programs will be quit”. If they have I can’t find it. I have tried using Automator to detect when the screensaver starts but that fails. I do not want an action to happen when I leave this chair. I’m sure this will be possible with Applescript but I’ve not gone there. But given the number of apps that sync I’m really surprised I cannot find what must be a winner. Seen anything that does this?