How to connect two NAS stations in network, one original data and one copy

[bmudde]

Hi there,
seeking help on configuration of two NAS units. DS211j and DS212j. Both with 2x 2TB harddrives.
DS211J is "openend"thru router for internet access. The second one is jut on network.
Configuration on both is now raid 1.
what i'd like:
- One unit full capacity available for data, and the second unit full copy of the first one. automatically.
- drive tot do so, make full 4TB data approachable from internet or for mobile applications.
- alternatively: how can i make the second NAS internet reachable, without having to change settings in Router.. etc

Please advise

- what is the best option to achief objective.