So if I get you right, 188.8.131.52 and 184.108.40.206 are not just L2TP servers but general purpose servers?
If so, you'll need to use policy based routing - mark packets to UDP port 1701 with a specific routing-mark using /ip firewall mangle
rules, and create the two route-pairs above with that routing-mark
/ip firewall mangle
add chain=output action=mark-routing protocol=udp dst-port=1701 new-routing-mark=l2tp
add routing-mark=l2tp dst-address=server1 distance=1 gateway=gw1
add routing-mark=l2tp dst-address=server1 distance=2 type=blackhole
add routing-mark=l2tp dst-address=server2 distance=1 gateway=gw2
add routing-mark=l2tp dst-address=server2 distance=2 type=blackhole
If you use fasttracking, the whole task becomes slightly more complex, see details here
But if the Mikrotik itself does not need to contact the two servers for anyting but the L2TP tunnels, you can use /ip route rule
to choose the routing table upon src-address
combination instead of the mangle rule. Route rules do not interfere with fasttracking.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace each occurrence of any public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.