Dualboot on an external hard drive
You can actually divide external drives into several partitions. 

To do so you just need to add a partition table to the drive using Disk Utility. Unfortunately that will render existing data on the drive unreadable by conventional means.

So in essence you'll need to copy your data to a backup, add the partition table, format the macOS partition and copy back your data. Then you can format the Windows partition and install Windows.