Safe to delete old "Macintosh HD - Data" partition?
It’s important to be certain that you’re deleting the correct data volume since they’re named identically:

In Terminal run the following command: diskutil apfs listVolumeGroups.
Find the volume group that contains both a System and a Data disk. That’s the one you’re going to keep.
The remaining volume group, which only has a Data disk, will be erased. Verify its name (“Macintosh HD - Data”). Note its device node (it will look like disk8s9 except the 8 and 9 will be different). Be sure to be VERY careful about writing this node down exactly.
Run the following: sudo diskutil apfs deleteVolume diskXsY, replacing the X and Y with the actual device node from Step 3. Again, be very careful with your typing here.