I have had a dual-screen setup (27'' iMac + 27'' thunderbolt display) for a while now; having recently acquired a MDP->HDMI adapter I'm struggling with mirroring the secondary display onto my TV.
The screenshot above shows an arrangement where the desktop is extended across all three screens. Now, if I select "Mirror Displays" then I get all of the displays mirrored i.e. same image on my iMac, thunderbolt display and the TV, which is not what I'm after.
Is there some obvious setting I'm missing? Can this be accomplished without third-party software?