GW_IP_1 ---                                    /--- R1
            |-- my_main_router_NAT_firewall ---|
GW_IP_2 ---/                                    --- R2

-t nat -A PREROUTING -p udp -s GW_IP_1 --dport 500 -j DNAT --to R1:500
-t nat -A PREROUTING -p udp -s GW_IP_2 --dport 500 -j DNAT --to R2:500