No problem, we do it all the time; some of our machines have more than a dozen instances running. One needs in essence a separate database, a separate installation tree and configuration file and a different port.
Assume, you will name your new instance "oacs-5-10". Create in the PostgreSQL cluster an additional database with this name (see below), create a separate configuration file for the new instance with a new "serverroot", "db_name" and a different port (when you want to run both versions at the same time). Install OpenACS 5.10 in a tree different to your 5.9 installation (e.g. /var/www/oacs-5-10). When you use a configuration file like [1], changing the Tcl variable "server" and "httpport"/"httpsport" should be sufficient.
To create an additional DB, Assuming, you have already a db user "nsadmin", you can create the additional database with commands like the following (where you will have to adjust the PG* variables according to your installation)
DB=oacs-5-10
PG=postgresql13
PGBIN=/opt/local/lib/${PG}/bin
sudo -u postgres $PGBIN/createdb -U nsadmin $DB
sudo -u postgres ${PGBIN}/psql -U nsadmin $DB -tAc "create extension hstore"
[1]
https://bitbucket.org/naviserver/naviserver/src/master/openacs-config.tcl
