SD-WAN and NAT with IP-Pool

I would try like that :

Create another IP Pool with the other public IP address from WAN2 (I don't know if One to One type will break current internet access for WAN2, I would also try also with Overload)

Change outgoing interface from WAN1 to SD-WAN in IPv4 policy

Set Dynamic IP POOL with both IP POOLS created.

MOVE this rule above existing IPv4 internet access rules so this will get hit first . 

TEST the rule and test the internet access for the company.

for testing you can try this on cli 

diagnose sniffer  packet  wan2 "tcp and port 25" 4

or

diagnose sniffer packet "tcp and port 25" 4    - for all interfaces or diagnose sniffer packet wan1 wan2 "tcp and port 25" 4

There is another option for SD-WAN , search on youtube "SD-WAN Rule Improvement: Load Balancing | FortiOS 6.2 " - but you don't have control over which interface traffic goes as this relies more on SLA's. It worth checking.  

P.S. - update public DNS records , SPF record to white list all your PUBLIC IP addresses from WAN 1 and WAN2 . 

Kind regards, 

Adi