partition hard drive with OS for each
I have an iMac (late 2015) running High Sierra, 1 TB HD, 1.6 Ghz Intel core i5, 8 G RAM. I would like to partition the hard drive in such a way that 1 partition boots into High Sierra and the main partition boots into Mojave when it comes out. So for now I would like 2 partitions, each able to boot into High Sierra. Is this doable and if so, how? I don't have any original install disks, as the iMac came with El Capitan, I think, and I have upgraded as they came along.
iMac (21.5-inch, Late 2015), macOS High Sierra (10.13.5)