Hi davizo70!
http://forum.mikrotik.com/t/best-method-to-load-balance-2-internet-line/37337/1
http://wiki.mikrotik.com/wiki/NTH_load_balancing_with_masquerade
I found these two examples quite informative but unfortunately i have not managed to successfully even make a simple load balancer from a RB750 for two adsl 4mb lines. Hope this helps and if i do come right i will post up my config here 