Any tips for running second Macbook air

[India Four Two]

Consider two separate Backblaze accounts:
https://www.backblaze.com/cloud-backup.html

combined with two bootable backups to external drives using SuperDuper:
https://shirt-pocket.com/SuperDuper/...scription.html

and for complete "all belt and braces" security, TimeMachine backups.

Some kind of external backup is doubly important if you are using an SSD rather than a spinning disk - ask me how I know!