The web interface for Pantheon allows you to create a backup for an environment, which saves a snapshot of code, database, and files. To do this through the Pantheon CLI, Terminus, you simply run:
terminus backup:create [site].[env]
But if you want to just back up a specific element — just the database, for example — you can pass in the element like:
terminus backup:create --element=db [site].[env]
Backing up two elements doesn’t appear to be supported, however. I tried:
terminus backup:create --element=db --element=files [site].[env]
but that only took the last element that I passed in, the files.