I have time machine saving onto two usb drives.  The backup is 4.6TB.  Each drive is 6TB.  Why is time machine only updating one drive?  (The other has been the same for two weeks.)

I have time machine saving onto two usb drives. 
The total size of the files being backed up is 4.6TB. 
Each external USB drive is 6TB.   So I have 12 TB of space for Time Machine.
Why is time machine only updating one USB drive?  (One drive has been the same for two weeks.)
I have selected both drives in the Time Machine settings panel.
My understanding was that it would do a full backup, and then incremental backups of changed files, getting rid of old copies as needed, and alternate between the two drives.
But it looks like drive A has one complete backup, dated two weeks ago, and drive B is the only one getting action.
There is a red icon on drive one that says the drive needs 120GB more space to do a backup -- which is strange since the whole backup set is only 4.6TB in size, and the drive is 6TB in size...

You could try a clone instead.
Clone  - Carbon Copy Cloner          (Often recommended as it has more features than some others)
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Clone  - SuperDuper
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