Make Time Machine use two drives from Terminal

My desire is to have two identical Time Machine backups on two different locally attached drives. My first thought was a clone via Carbon Copy Cloner, but it won't do it, because Apple won't let third parties copy the filesystem that is being used for Time Machine backups (that's what the "error"-message says before I can even start the clone).

The reason why I need a command line approach is because I want to include the two backups in task-chain in Carbon Copy Cloner.

Everything I have been trying in that direction fails (no help found online so far), as Time Machine would always start backing up to the same drive over and over again. So what I can say so far: tmutil is not the way to go, no matter what options I choose. But maybe I missed something?

If I manually start the backup by clicking "backup now" in macOS (High Sierra 10.13.6 by the way), Time Machine will backup to drive 1 first, if I click it again (after backup has finished), backup to drive 2. By the way: same bevaviour if I set a schedule via Time Machine Editor.

So I am looking for a Terminal-way to imitate the macOS-approach to start the backup, as it seems this is the only way to make Time Machine use two different drives. tmutil only uses the last destination, again and again. Even if I setdestination to drive 2. But again: maybe I missed something.

I don't think using terminal to configure Time Machine to back up to 2 different disks is necessary. If you have Time Machine setup currently to back up to one external disk, you can go back to the Time Machine preferences in System Preferences and select a different drive to back up to. After doing this you will be presented with the option of using both drives for Time Machine backups or change the existing drive. If you select the option of using two separate drives, the Time Machine backups will alternate destination drives for each backup. At least these are the options I am presented with, while using two separate 3 tb Time Capsules.

Thank you both for your effort and your help! I should have clarified my setup first, sorry for that! I already had both drives locally attached and selected as destinations in system preferences. I DO need the terminal command I have explained in my initial post: I want to run a short shell-script, which I can attach at the end of a finished Carbon Copy Cloner task. I have tried jaumes approach now and it worked: tmutil startbackup --destination <disk id 1> --block; tmutil startbackup --destination <disk id 2> --block. Why didn't I see --destination before?? I ran man tmutil a couple of times. Thank you!

Edit: someone else replied in the meantime: thank you! I will try the --rotate approach as well. But don't you use --block ? Otherwisee it just jumps to the next command and doesn't wait for the backup, or backupd, to actually finish. I thought that would lead to an error.

I set up multiple destinations and just tell the system to rotate, it then chooses another drive if it’s mounted:

 tmutil startbackup --rotation --block

I often chain these so I get several connected drives both capturing a backup interval now(ish)

 tmutil startbackup --rotation --block; tmutil startbackup --rotation --block

I just use the GUI system preferences to add all my destinations and then I use terminal to mount and unmount the volumes that I want included/excluded from any potential rotation backup event (or normal time to backup event - which defaults to the rotation algorithm as well). I’ve just set the destinations and forgotten them basically.