What is the older version of macOS you are trying to install beside your Monterey installation?
FYI, you won't be able to launch & run an older OS installer while booted into Monterey since Apple doesn't allow for that option in order to protect people I guess. You will need to create a bootable macOS USB installer using the instructions in the following Apple article:
Create a bootable installer for macOS - Apple Support
Also, if you are going to be installing macOS 10.10 to 10.12, then you cannot install the older OS into a new APFS volume since they do not understand the APFS file system (technically 10.12 can read an APFS volume). Installing macOS 10.10 to 10.12 would require splitting the drive by resizing the existing Monterey partition and creating a new partition with the file system "MacOS Extended (Journaled)".
If you are installing macOS 10.13 - 12.x, then creating a new APFS volume and installing macOS 10.13 - 12.x into the new APFS volume is the optimal method since it doesn't modify the physical partition structure (APFS volumes behave a lot like partitions, but shares the same storage pool of the parent Container).
FYI, I do not recommend partitioning any drive for any reason. Partitioning is always risky to the existing data on the drive, plus people usually discover one or both partitions are too small......which means they need to start all over again. It is much better to install another OS onto an external USB3 SSD. If you do use multiple partitions, then make sure to first have a good backup of the existing volumes.
When installing macOS to an external SSD, just make sure you erase the whole physical drive as GUID partition type and a file system compatible with that OS you are installing. Many drive manufacturers tend to have odd drive layouts from the factory which can cause issues with an OS (especially when booting from the drive), so it is best to erase the whole drive so a new partition table is created. Within Disk Utility you may need to click "View" and select "Show All Devices" before the physical drives appear on the left pane of Disk Utility.
If you are intending to install macOS 10.10 Yosemite onto an external SSD, then you need to use Disk Utility to manually partition the whole physical drive using the instructions in the following article:
https://eshop.macsales.com/tech_center/formatting/Mac_Formatting_6-10.pdf
