Possible to use the same ssd for a Windows and a Mac laptop via different partitions?
I think this might be a better fit for Super User as it's mainly about PC/Windows rather than Mac. 

However, as a rough guide -
you'd do far better to clone the Mac's boot &amp; recovery partitions to a spare drive using something like Carbon Copy Cloner then let the Windows installer [from a PC] just break the whole darn thing - which it will, it will insist on NTFS &amp; will probably set up its own EFI &amp; recovery partitions; and Windows won't boot easily if it's not at the beginning - then use the Mac to restore the 2 Mac partitions afterwards to the end of the disk.  This will likely need 3rd Party software, such as Paragon Hard Disk Manager   

Windows also is not truly portable like macOS, so each time you move it to a different machine it will either crash/fail or require re-authorisation from Microsoft. Windows is also not too keen on booting from a removable drive.

Ultimately, I think buying a second SSD would be simplest &amp; safest.