How do I backup 2 external hard disks to 1 external hard disk?
This can even be done in Time Machine:

Choose the 5 TB HDD as only Time Machine backup drive (OK, my disks are smaller  ):



Then exclude your internal drive by adding it to the excluded items - the backup drive is excluded automatically:



The final result:

You get a folder Backups.backupdb with one folder vm (the name of my Mac) containing folders (with date names representing the backup date) and each with two folders representing the two drives (Data1 and Encrypted). I added and modified files successively on both volumes and initiated a backup after each step.





Needless to say: it's recommended to backup your main drive/system drive also!