This feature is available in DRBD 8.0 and later.

To enable dual-primary mode, add the allow-two-primaries option to the net section of your resource configuration:

resource resource
  net { 
    allow-two-primaries; 
  }
  ...
}

When a resource is configured to support dual-primary mode, it may also be desirable to automatically switch the resource into the primary role upon system (or DRBD) startup. To do this, add the become-primary-on option, available in DRBD 8.2.0 and above, to the startup section of your resource configuration:

resource resource
  startup { 
    become-primary-on both;
  }
  ...
}

After you have made these changes to /etc/drbd.conf, do not forget to synchronize the configuration between nodes. Then, you can use drbdadm adjust resource (on both nodes), and afterwards, drbdadm primary resource (again, on both nodes).