Diverter valves are similar to BOV's. They discharge the air back into the system, instead of into the atmosphere like regular BOV's.

Diverter valves are what most stock cars have that come turbo'd from the factory.

I'm not usre why'd you want two blow off valves someone else my know. But you dont' need two blow off valves for 2 turbo setups.

Blow off valves discharge air from the charge pipe (from intercooler [if there is one] to the throttle body). Most of the time it's released into the atmosphere.

I believe super charged systems have blow off valves or diverter valves.