Two operating systems on same drive?

[Agincourt]

I want to know whether I can keep two operating systems on the same drive. I've attempted to partition and install a second OS on the ~40 GB space I reserved specifically for 10.15 but it overrode and replaced the 10.13 that I wanted to keep. I've a few old programs that need 32 bit support, thus I need to keep High Sierra. However I've a few programs which demand 10.15 or later, so I want to be able to start up under either of these operating systems. Only other option is an external drive, but that would be painfully slow and unsightly.

Anyone know how this can be done on one drive?

 

[Mike Boreham]

https://support.apple.com/en-gb/HT208891

Works very well. Note use space sharing APFS volumes in same Container, not separate fixed partitions.

EDIT I have never done it with High Sierra but High Sierra is APFS should work.

What you tried should have worked. I can only assume you may have selected the wrong partition to install the second OS on.

 

[DeltaMac]

Yes. All you need is separate volumes for each boot system.
I have shared on this site before that I have a single drive with (now)14 Mac systems, each on separate partitions or apfs shared volumes. Allows me to use that one drive to boot any OS from Leopard to Ventura.

Note that you could keep your 32-bit apps with Mojave (macOS 10.14), which should work as well as High Sierra for that.

Be careful about which volumes that you use for the OS install destinations. I take some time to make sure the names of those install volumes are distinctive, so you don't accidentally choose the wrong volume for the install.