Well, again I find that confusing, a bridge is functionality within the device, its either a switch or a router etc.
So I will take it as switches.
Suggest you read this guidance on how to use vlans and switches.
Having two bridges is also confusing, all one needs is ONE bridge.
Change your subnets to vlan77 and vlan78 for example.
The bridges have then no DHCP responsibility etc…
http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1