expansion or second NAS?

[ed]

Having a backup is certainly the best idea. You can also link both NAS together in order to access the data via a single connection. Having an expansion unit might cost similar to a new NAS. This is why people don't buy them. Having two NAS systems will allow you to set up remote backup tasks. So both NAS can backup up certain folders to each other automatically. This way you keep your data safe and accessible, while also having more storage space.

I hope this helps.