Hi,
this "Load balance two ADSL lines" have been discussed a lot in the forum up to now, and no one have ever posted a solution. It has even been discussed at the first MUM in Praha.
The problem is the session persistence. One session (like a IM session, a HTTPS session) must be handled by one WAN port. Otherwise the server on the other side will get confused if he gets requests from one client but over two WAN ports (with different WAN IPs/Gateways/Providers).
To create two default routes is no solution. We need a solution for NAT'ed WAN connections like two PPPoE connections.
See other postings:
http://forum.mikrotik.com/viewtopic.php?t=6461