I understand the need for 2 or more bridges. One can set many different things in the two bridges. The separation is clean, and the link can be specific for protocol or speed.
I know we moved away from this multi-bridge model for VLAN separation, as the bridge/switch handles the VLAN now. But there is more than VLAN to separate.
I used to do this often in a Fortigate with "VDOM". Separate firewall's with virtual interfaces between them. Actually this is possible in RouterOS with MetaRouter. But the MetaRouter is too big to have multiple. (It's like having virtual machines in Windows (Virtualbox, VMware, ...) where you actually would like to see virtual application in one OS (like APP-V (Softgrid), or VMware ThinApp), that is more what VDOM in the Fortigate does.) There are really multiple cases for this multiple "VDOM"/"MetaRouter" design, but MetaRouter requires too many resources.
But here the question is just multiple bridges (only one can be HW offloaded per switch chipset) , with virtual interfaces between them. Would also be nice to connect to HW offloaded bridges, without losing HW ethernet interfaces.
I just tred to do it with "virtual ethernet" but found no way to connect any virtual interface to 2 bridges at the same time. If we have such virtual interface it could be done.
Or should we use a tunnel. What would be the better choice? EoIP, MPLS/VPLS, VxLAN, SSTP tunnel, PPtP tunnel, and other ways to directly connect the 2 LAN's ?