First, my own personal pet peeve, please do not double NAT, it is never worth it, it will make plants wither, your day will always be gloomy, and there is a risk of hair loss. In all seriousness, it is a bad idea, it will lead to issues at some point, and the risk of hair loss is real (you might pull it all out in frustration).
Now on to business...
Either way you will need to make sure your rules/policies allow the traffic over either WAN, I presume you have already done this half since you managed to get WAN1 working.
If this scenario is 100% failover (WAN2 need to work only when WAN1 is down), then adding route monitoring to the WAN1 default route will be sufficient. If your routes were added via the interface automatically, you will need to disable "automatic default route" on the interface and manually write the default route for WAN1. In this scenario you will find that traffic to your WAN2 IP fails until WAN1 is brought down, at which point WAN2 starts working.
If you wish to use both WANs at the same time, you will need routing rules specifying that traffic coming in over WAN2 should return over WAN2. This will entail a second routing table where the order of the default routes is reversed and a routing rule directing return traffic destined to WAN2 to that table. Take note when writing the routing rule, that the routing rule matches based on your main routing table for destination interface, so your destination interface will need to match the first match on your full routing table (check the all routes checkbox in status routes to see the full version) for your WAN2 IP.
Good Luck!