Two ways.
1. If the new server can peer with the old one, just run both servers for a while. The new one will gradually pull data it needs from the old one as clients request it. This method is best if the new server already has data cached that the old one doesn't.
2. For a more immediate clone, stop both caching servers, run this command in Terminal on the new server, then start the new caching server up again. Replace admin-user with the name of an administrator account on the old server, and old-server with the name or IP address of the old server.
$ sudo rsync -a --delete admin-user@old-server:/Library/Server/Caching/Data/ /Library/Server/Caching/Data/
The sudo and the trailing slashes (after Data) are important. This command replaces the new caching server's cache with the contents of the old one, assuming both of them store their caches in the default location on the boot drive. If either one uses a different location, modify the command accordingly. Be sure to copy only the Data directory and not the entire Caching directory.