Могу ошибиться но как-то так
/ip route rule
add src-address=IP_WAN1 table=WAN1
add src-address=IP_WAN2 table=WAN2
add action=lookup-only-in-table routing-mark=WAN1 table=WAN1
add action=lookup-only-in-table routing-mark=WAN2 table=WAN2
/ip route
add distance=1 gateway=WAN1_GATEWAY routing-mark=WAN1
add distance=1 gateway=WAN2_GATEWAY routing-mark=WAN2