Some non-portable applications can be "dual installed" on more than one OS in the same spot. If they don't put any files in shared folders or dlls in system32.. that type of thing. I used to have multiple windows OS on a drive with some apps installed under both to save space(drives were a lot smaller and more expensive then.) Some things worked pretty well. Others were more trouble than it was worth.
If the program basically has all it's code files in the install folder and uses the registry for settings, you can install under one OS, boot into the other then install again. This has the registry settings for both OS while only having one copy of the code files. But the larger the program, the more the space savings, then the greater the likelihood it will do stuff like install dlls in shared folders etc..
Also it requires the programs be installed on a partition visible from either OS when that OS is booted.
Some programs like news readers and web browsers worked pretty well, but that was back before the Local and Roaming folders for settings. Most apps that needed an install kept settings in the registry.
Uninstall was more of a hassle. You had to uninstall under one OS then use a registry cleaner in the other to remove settings for files that no longer existed.
It's only worth considering if you have some monster program and are tight on space. Otherwise just use portable apps.