Merging two APFS partitions when one is time machine?

I found four similar questions, but they weren't exact duplicates.  I did get an impression from them that it isn't possible, but since my situation is slightly different, I thought it worthwhile to ask.
Blank is just that: no files except the unnecessary .fseventsd and Spotlight file.  But Christopher is a time machine backup that is almost full (4.78TB).  I would like to make this a single APFS container without losing any backup sets.  I have nothing else big enough to copy its contents (nor would I enjoy the hours it would take at USB 2.0 speeds).