A design question about interconnecting two sites by VPN

I have main site with data center and office users and a secondary site with office users about 600 km apart. The office users at 2nd site are connected by VPN to the data center at the main site. Link to scheme

Now the management is thinking about building a data center at the 2nd site to use for DR of critical system systems. The requirement is if data center at first site is unavailable to switchover virtual machines to DR site so that users at both sites and the internet users maintain ability to work with criticals systems. I am an infrastructure architect tasked with proposing DR soluting.

How should the 4 network objects (main dc at site1, users LAN at site1, DR DC at site2, users LAN at site2 ) should be interconnected by VPN? Should there be 4 sets of routers to connect each objects? Or should the external network into a separate be separated from the data center so there is only one set of routers per site ?