Ok, sorry but this is the only way!
Anyway, if this answer solves your need, please accept it for the other people of Community.
Ciao and happy splunking.
P.S.: Karma Points are appreciated 😉
in this case, the easiest way is to copy the search and configure the same alert in the other instance.
You could also copy the alert from $SPLUNK_HOME/etc/apps/<your_app>/local/savedsearches.conf, taking the stanza of the alert to clone and copying it in the other instance, but in my opinion it's easiest to copy the search and create the alert.