UTM-to-UTM RED tunnel

 Site-to-site topology with two UTMs ver 9.4x.. Followed Sophos resource to build my first RED tunnel with my IP subnets and all worked well, no problems. Resource: https://community.sophos.com/kb/en-us/120157

Now I want to build a RED tunnel for a layer 2 application and I need to bridge both LANs via the Tunnel. I cannot figure out how to bridge LAN port to its respective virtual interface (i.e. REDS1, REDC1) on each UTM, assuming this is the necessary step at least conceptually. Any guidance from the community will be much appreciated...