npm i sync-zsh-history -g
create a file under the synced directory of some cloud storage service, let's say ~/dropbox/.zsh_history
synczshistory ~/dropbox/.zsh_history
it will merge ~/dropbox/.zsh_history and ~/.zsh_history, remove duplicated commands, and write the results back to both, then you can merge it in another machine.
wget https://raw.githubusercontent.com/mrzzcn/sync-zsh-history/master/com.github.mrzzcn.sync-zsh-history.plist
# Before this, you may want to change trigger time and log file location.
launchctl load -w com.github.mrzzcn.sync-zsh-history.plist