The Samsung T5 SSD is already partitioned out of the box with a standard-access partition (that anyone can access) and a secure partition (which requires you to install the Samsung software to read and write to).
Did you wipe and repartition the whole drive before you made the Windows backup? In the scenario you're talking about, you would probably need to defeat the T5's built-in encryption software and just repartition the whole drive from scratch. The Windows partition should be NTFS if possible, and the macOS partition should be APFS (encrypted). One drive, at least two partitions, and the backup for each OS (Windows and macOS) goes on their respective partition. Do the partitioning first to set up the drive (warning: this may destroy all existing data on the drive!) and then create your backups in the respective OSes. See if that works and let us know if you have any further questions.
It also wouldn't hurt to check the Samsung T5 documentation (downloadable from the Samsung support website) to make sure that you can just wipe the whole drive and repartition as you like. This drive is set up with Samsung's proprietary security software out of the box (as mentioned above), which applies to the lion's share of the drive. I'm not sure how much of that is firmware-based. A total wipe and repartition might or might not be possible with that drive.
