what you want is is bounding, but as I see your setup, you plan to have just one RB and use 2 wlan cards? then the MPLS OSPF etc are still overkill and for no use. However,If you have 2 RB boards, it might be a solution. But still, I would try not to use it. The less logic the radio have, the more resources are left for sending packages. Its also much easier, than hassle with MPLS, routing etc etc.
I would consider something from http://wiki.mikrotik.com/wiki/Manual:Interface/Bonding
as long as your plan is a "point to point" link A-B-C-D etc etc.
If you plan to do a ring,mesh and so on, the MPLS etc would be nice. eg A-B-C-D-A + B-D. MPLS works well as a "non defined cloud" and random connections. Not a difined from point A to point C trough point B.