Only to keep things as together as possible, the proposed solution by sindy was tested as working in a very similar setup, here:
http://forum.mikrotik.com/t/port-based-routing-to-2-identical-ip/180504/1
At the end a couple of blackhole routes were needed in addition to avoid the second device to be reached through the main ADC route in case of a broken connection (cable pulled out or device off) to the first.
@anav
Op there posted the final configuration reported as fully working.