how to select a second disk to be backed up on time machine

I have always backed up my macintosh SSD on a 6 TB HDD using time machine. I now want to add another HDD to be backed up on my time machine disk as well.

I can see where the system disk is specified and also specify the disks that I do not want backed up. But I cannot find how to backup the second (or third etc.) disk.

Please advise:

1) if this is possible

2) how to do this.

Thanks

Barry