I used to run several different apps from one database. I even had two CMS systems in the same database at times.
You can manually install another WordPress instance in a subdirectory and use the same database credentials as the original install but change the table prefix.
Just do a backup of everything in case anything goes wrong and be very sure to use a different table prefix.
Your other option is to convert the existing WordPress to multi-site but if you don’t need to run multi-site then I’d discourage that.
In either case Be mindful of space limitations…
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Also, you don’t need the database to be on the same host if you have a host elsewhere that could donate a database. The donor host needs to allow the donated database to be accessed from outside… quite often a cPanel setting or allowed by default.
I often use this ‘trick’ to migrate sites from one server to another.