Sure lets say,
WAN1 gateway is 64.24.33.22
WAN2 gateway is 24.165.24.122
IP of NVR is 192.168.1.30
Assuming: Wan1 is primary and and Wan2 is only used if WAN1 is unavailable.
This ensures any traffic ORIGINATING from the NVR will go out WAN2.
How traffic reaches the NVR is up to how you setup port forwarding.
Ensure that the DYNDNS address or WANIP you give external users is affiliated with WAN2.