Why cat the two files together first?
Multipart archives are designed to work like normal as long as all parts are present in the same directory.
You don’t need to do anything special, nor use extra steps (like cat) to create a new .zip file to then later extract.
It’s as simple as this:
7z x manjaro-unity7-22.0.r202212021811-221202-linux60.iso.zip
Done.
You just need to make sure all parts are present in the same folder.
Here’s proof:
All files of the multipart archive in the same folder:
ls -1
manjaro-unity7-22.0.r202212021811-221202-linux60.iso.z01
manjaro-unity7-22.0.r202212021811-221202-linux60.iso.zip
Extract the .zip file:
7z x manjaro-unity7-22.0.r202212021811-221202-linux60.iso.zip
Now the .iso is fully extracted.
ls -1
manjaro-unity7-22.0.r202212021811-221202-linux60.iso
manjaro-unity7-22.0.r202212021811-221202-linux60.iso.z01
manjaro-unity7-22.0.r202212021811-221202-linux60.iso.zip
And it even passes the checksum, using the SHA256 hash to confirm its integrity. 
Using cat to create yet another huge extraneous .zip file, and then extracting that new large .zip file is a complex method when you can just do it in a single command:
7z x manjaro-unity7-22.0.r202212021811-221202-linux60.iso.zip