Just FYI, but Q-in-Q is not really going to help. What he might be able to do is create a standard Q-Tag/Trunk between the switches (Q-in-Q not required). Then create a L2 Cobranet VLAN than spans both switches, along with a L3 VLAN that is just the Q-Tag/Trunk interfaces and enable OSPF on that for routing. You would then use the Routed VLAN for any L3 and the L2 Corbranet for all that equipment.
NOW, if that Corbanet VLAN, must get to everywhere else, then you will also need to assign an IP address ONLY on one switch, and then enable OSPF there as well. Basically make one side L3. This would allow L2 between the switches for Corbanet to work properly, and route on one side to rest of network.
Hopefully this makes sense to you. This is just basic L2/L3/VLAN stuff. Good luck.