I have achieved, without partitioning or using virtualisation software, two different macOS installations; Mojave & Catalina on one Mac.
How, you may ask? First I intercepted the Catalina installer, interrupting it & stopping it from going forth to do its deeds. I then used DiskMaker X for Mojave to create a bootable Catalina installer drive.
Next, into Disk Utility & add another volume to the existing Mojave volume. Restarted from the installer drive & told it to install Catalina on the new volume. This eventually happened, taking approx 90 minutes.
After booting into Catalina I set up my secondary (mostly used for testing) Apple ID. That done, I then selected the Mojave volume as the start-up source & now, after a slow start, it’s working OK.
My iMac has a 1.03TB Fusion drive & perhaps the slower bootup was the machine reallocating different apps & processes from the HDD part to the 32 GB SSD component.