Yes, you need
1. Routing > Routing tables > Add routing table (ex, alt_wan2)
2. Add into this table route for all-nets to wan2 (if need, don't forget about gateway)
3. Routing > Routing rules > Add PBR
lan/yourhost wan1/all-nets, forward alt_wan2, return main
4. Interfaces > Interface groups
Add group "wans" with wan1, wan2
5. On Rules > IP rules > lan_to_wan1, replace wan1 to wans in NAT rules