MacOS

Hey there tuomas240,

Welcome to the Apple Support Communities!

We understand you're wanting to know more about creating another disk with El Capitan installed on your machine with macOS Big Sur.

If you're needing a bootable installer, you can absolutely do so with our steps here, which also include download links for each of our available OS X or macOS versions:

How to create a bootable installer for macOS - Apple Support

If you'd rather have a separate volume where you can then select either macOS Big Sur or OS X El Capitan in selective startup, try our steps here:

First: Partition a physical disk in Disk Utility on Mac - Apple Support and make sure to use Mac OS Extended (Journaled) as the format for the partition you're wanting to use with OS X El Capitan.

Next: How to reinstall macOS - Apple Support

From there, you will use Selective Startup to choose which disk to load your Mac into:

Change your Mac startup disk - Apple Support

Let us know if you have further questions!

Have a great rest of your day!