I boot System 7.1 and System 7.5 on my Mac (due to software compatibility reasons) using System Switcher (or something like that) and two System Folders (the less-used one has got a different name) all in a single partition. People have always done just that to dual-boot System 6 and System 7 back in the day. It's only when you're working with operating systems with different architechtures (like the Macintosh operating system alongside Linux or A/UX or something like that) that you need to start worring about partitions.