I don't know how or if it's possible to set up two SMTP servers, but I know that may ISPs block SMTP traffic that is not directed to one of their SMTP servers. You could try picking just one SMTP server, and find out if it can be conacted on a non-standard port. A lot of SMTP providers allow for this.
If you can configure a single SMTP server on a non-standard port, you should be able to conatct that SMTP server from anywhere on the internet because the traffic won't be blocked (at least not port-based blocking, which is what most ISPs use).
So in a scenario where WAN1 is the ISP who owns the SMTP server and WAN2 is a diferent ISP that blocks standard SMTP traffic...
1) If both WANs are working, SMTP traffic goes out WAN1. No problem.
2) If only WAN1 is working, SMTP traffic goes out WAN1. No problem.
3) If only WAN2 is working, SMTP traffic goes out WAN2, but is not blocked because it is on a non-standard port. No problem.