You have to do policy based routing via the mangle table
Have a look at these links as a guideline
http://wiki.mikrotik.com/wiki/Policy_Base_Routing
http://wiki.mikrotik.com/wiki/Load_Balancing_over_Multiple_Gateways
http://forum.mikrotik.com/t/policy-based-routing-using-two-uplinks/51312/1