How do you can install mojave on another disk apfs toghether Catalina?

This can only be done on a computer that shipped with Mojave or earlier that is compatible with both.

1. Backup your data. (note if you don't clone your data, and use Time Machine your backup will only be recoverable under Catalina).
2. Backup your data to a second separate source.
3. Open Disk Utility
4. Use Internet restore, command-option-shift-R
5. Use Disk utility to add a partition. Rename the empty partition Empty. Be sure your partition has at least 50 GB free, and in making a 50 GB partition, the partition Catalina has is not shrunken to a point where it is over 85% full.
6. Quit Disk Utility.
7. Run the installer that came with the Mac. If the Mac came with High Sierra, or Mojave it should work. If it came with Sierra, it may not work, unless you reformat the empty partition as HFS Extended Journalled with Disk Utility.
8. Once you installed the operating system in question, if it was not Mojave, you can upgrade to Mojave using these directions on the empty partition.

If you can't make a big enough partition, or if you can't format the drive, recover your backup.