Currently, Mikrotik can not elementary thing - connect the two bridges. (http://forum.mikrotik.com/t/how-to-connect-two-bridges/97236/1)
I could not believe it, but Mikrotik really can not do it!
With Virtual interface without tagging it can be done.
Also, it can be done with vlan interfaces and vlan tag management, but Mikrotik do not have any manual vlan tag management! (http://forum.mikrotik.com/t/bridge-and-vlan-tag-management/97076/1)
I have already create post with feature request (http://forum.mikrotik.com/t/feature-request-switch-like-vlan-functionality-for-rb-w-o-switch-chip/97142/1)