How to install High Sierra on a second partition on a MacBook which already runs Mojave?
There is a simple solution. 


Using Disk Utility create a second Partition for the High Sierra:


Probably your disk is in APFS, so simply use "+ Volume" button at the top left of Disk Utility app, give new Volume a name (for example "Test HD").




  If there is a need to install OS that can't be installed on APFS (for example Sierra and earlier) use "Partition" button to create separate HFS+ Volume.
  
  





Then, create a USB flash installation disk of High Sierra using terminal:



  USB disk needs to be formatted with a GUID partition table (GPT). Use Disk Utility to erase the USB disk and choose GPT for the partition scheme.


sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume


Where


/Applications/ - path to the Installation of High Sierra
Install\ macOS\ High\ Sierra.app - Name of the distributive
Contents/Resources/createinstallmedia - Command for installation
--volume /Volumes/MyVolume - USB flash drive path


Information from Apple:
https://support.apple.com/en-us/HT201372




Now install High Sierra:


Turn off your Mac. Hold down option and turn on Mac. Choose your USB Installation Media (it will have High Sierra Icon). Follow the instructions. 
Choose High Sierra Volume ("Test HD"), that you previously created in the 1st step and finish installation of High Sierra. 

Switch between systems using option on startup.