I want to use Disk Utility, to create Disk Images of the 2 partitions...

OP wrote " I want to use these discs as backups before erasing and installing MacOSX Sequoia. "

Use Time Machine Backup Utility  you maybe able to Restore the Deleted Files 

To truly protect your non replaceable Data

Have a 3-2-1 Rescue Plan in place and always current

3 Backups using 2 methods and 1 off site incase of natural disaster or un-natural disaster.

Each of the above should be done to a Dedicated Single Purposed External Drive 

Below link is intended to augment what TM Backup does 

https://bombich.com

Creating legacy bootable copies of macOS

FYI, if this is an Intel Mac, then it may be possible to make an archive image of the macOS boot drive using Carbon Copy Cloner as mentioned by @Owl-53, but with the T2 Intel Macs (2018-2020) it may or may not work correctly when you go to restore the bootable image. If you have an M-series Mac, then just forget about making a bootable archive image of the boot drive because all of the new security features will cause a problem if you try to restore such an image. Even the developer of CCC posted on their site about 5 years ago that making bootable clones of a macOS boot drive is no longer recommended due to the enhanced security features with both macOS & the Apple hardware.

https://bombich.com/en/kb/ccc/6/cloning-macos-system-volumes-apple-software-restore

Save yourself many headaches and only concentrate on backing up the data. If you have any need to reinstall macOS, then you will need to restore the data from a backup.

Also, Disk Utility is an absolutely terrible app which cannot even perform its most basic features of erasing & repairing volumes. Disk Utility has never been able to properly image a macOS boot drive, nor any large volumes or folders. To do such things I have always used CCC and never had any problems.

HWTech wrote:

with the T2 Intel Macs (2018-2020) it may or may not work correctly when you go to restore the bootable image

I have for testing purposes made a "legacy bootable" .dmg from a cleanly installed Mojave, Catalina, Big Sur, Monterey, Ventura, Sonoma and Sequoia for my T2 Intel Mac mini 2018 and all of them can successfully be cloned back to a bootable external USB volume.

I have used OWC Dual drive dock and OWC Envoy Pro mini for this but maybe other brands might not work as well.

Matti Haveri wrote:

HWTech wrote:

with the T2 Intel Macs (2018-2020) it may or may not work correctly when you go to restore the bootable image

I have for testing purposes made a "legacy bootable" .dmg from a cleanly installed Mojave, Catalina, Big Sur, Monterey, Ventura, Sonoma and Sequoia for my T2 Intel Mac mini 2018 and all of them can successfully be cloned back to a bootable external USB volume.

I have used OWC Dual drive dock and OWC Envoy Pro mini for this but maybe other brands might not work as well.

Thanks for confirming that a basic cloning operation is still possible on the Intel Macs.

I figured it could create a bootable clone on an Intel Mac, but my concern is unforeseen side effects regarding the T2 security chip. For example, booting into Recovery Mode and being able to authenticate to the T2 chip to access the internal SSD or security features. I've seen enough bugs with the T2 chip in Recovery Mode that I would be very cautious. What happens if the admin password is changed after restoring the clone, can you still authenticate successfully with the T2 chip in Recovery Mode (this can only be tested if the internal SSD is erased & restored with a clone, otherwise the T2 chip may only pick up the information for the original OS on the internal SSD and not the restored clone)?

👍