If understand correctly (If I'm wrong give some more information):
1. You cannot use xxxxxxxxxxxx.sn.mynetname.net address to login at MikroTik router behind NAT, xxxxxxxxxxxx.sn.mynetname.net assign DNS for external IP only. But you can use portmap (port forwarding) on "ADSL box" like:
[external-IP-of-the-ADSL-box]:10001->[mikrotik-1-IP]:8291
[xxxxxxxxxxxx.sn.mynetname.net]:10002->[mikrotik-2-IP]:8291
(xxxxxxxxxxxx.sn.mynetname.net is equal to external-IP-of-the-ADSL-box)
[mikrotik-2-IP] and [mikrotik-1-IP] must be static
2. Use winbox to connect to xxxxxxxxxxxx.sn.mynetname.net:10001 to connect to router 1, or xxxxxxxxxxxx.sn.mynetname.net:10002 to connect to router 2.
Note that ADSL provider must give you not a private IP address; and don't forget to add firewall rule to use winbox:
/ip firewall filter add action=accept chain=input comment="Permit Management" dst-port=8291 log-prefix="" protocol=tcp