Partition an external drive

Yes, it is possible. You can use macOS Disk Utility. See Partition a physical disk in Disk Utility on Mac - Apple Support.

• Backup any data that you need from the disk.
• Use Disk Utility to erase the disk, and create two partitions - one is MacOS Extended Journaled (JHFS+), the other is exFAT.
• The JHFS+ partition is not readable/writeable by Windows.
• The exFAT partition can be read-written by both OSes.
• If you need to use the Windows partition for backups, it should be formatted as NTFS, but you will lose writability from teh macOS side.

If you need an example, please ask.

• In macOS, you can drag-and-drop files from one partition to another, when they are JHFS+ and exFAT, respectively.
• macOS can read NTFS, but by default cannot write to NTFS. Third-party software can enable this, but can be buggy.
• Windows can read-write exFAT and NTFS, but requires third-party software to write to JHFS+. I do not recommend third-party APFS software, because it is unstable.