Hi , it's a 3 years posts but i'm going to answer it !!
it's not so easy , but you can do it with OSPF routing protocol !! that's all
if you are not friend;t with OSPF , use static routing , but in this case you will loose redundancy , with OSPF you could have full duplex and redundancy in case of radio failure
Here is an example of how to do this http://wiki.mikrotik.com/wiki/Dual_Setup_with_OSPF
If you use an RB750GL at each end and 4 QRT's for example you would create a /29 for each link and run the QRT's all as Layer2 bridges.
On each RB750GL you set the inbound interface in OSPF to have a cost of 100. Outbound interfaces have the default cost of 10. This makes the traffic flow in a circular fashion. If one link fails the remaining link will then handle all traffic.
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