Can I ask Time Machine to backup only some data to a smaller disk? I have two backup hard discs for Time Machine: 1.5TB, and 1TB. My backup fits in 1.5TB, but Time Machine sometimes complains that it can't backup to the 1TB disk, because the space is not enough.

Can I ask it to automatically select only some directories to back up, instead of failing to back up? I still want everything backed up to the 1.5TB disc.

I'm not using the 1TB hard disc, anyway, so I would like to backup some data to it rather than none, in case both my computer and the 1.5TB hard disc fail.
You can exclude items from a backup to save space on your backup disk, or to exclude items you back up using another method.

Choose Apple menu > System Preferences, then click Time Machine.

Click Options.

Click the Add button , then select one or more items. You can also drag items to the list. To remove an item, select it, then click the Remove button . For example, you can exclude system files or the Applications folder from Time Machine backups using Time Machine preferences.

The backup disk is automatically included in the list of items that shouldn’t be backed up.

Click Save.

https://support.apple.com/kb/PH18847

Notice how this setting is for both timemachine destinations though. Excluded files will (eventually) be deleted on both destinations.