I hope these links help you.
Buenas, espero que te ayuden estos link.
https://wiki.mikrotik.com/wiki/Manual:Load_balancing_multiple_same_subnet_links
and the forum:
y del foro :
http://forum.mikrotik.com/t/2-wan-in-same-subnet-with-same-gateway/42216/1
saludos
Temorizador