I also add routing mark for firewall mangle (not required in 7.x)
/routing rule
add routing-mark=to_WAN_pppoe2 table=to_WAN_pppoe2
and adds a WAN connection address to handle outgoing connections via the backup (example, isp gw: 2.2.2.1 mikrotik ip: 2.2.2.2)
/routing rule
add src-address=2.2.2.0/30 table=to_WAN_pppoe2
for me it will be complete:
/routing table
add fib name=to_WAN_pppoe2
/ip route
add dst-address=0.0.0.0/0 gateway="backup_PPPoE_interface" routing-table=to_WAN_pppoe2
/routing rule
add routing-mark=to_WAN_pppoe2 table=to_WAN_pppoe2
add src-address=2.2.2.0/30 table=to_WAN_pppoe2
add action=lookup-only-in-table src-address="Specific_Machine_IP" table=to_WAN_pppoe2
…and similarly for to_WAN_pppoe1
This will ensure that no matter which gateway has a greater distance, both will be reachable from the WAN.