MacOS – How to mirror secondary display to a TV

You've pretty much got two options for two external monitors on a MacBook Pro:

A) Two Apple Thunderbolt Displays

This is the least flexible, but highest performing solution. Obvious drawbacks are the cost, and not being able to use your existing displays. This only works on Thunderbolt equipped Macs with a discrete graphics card (i.e. not the 13" MacBook Pro or any MacBook Air, nor the base Mini).

B) DisplayLink adapter

You can use one display directly connected to the Thunderbolt/Mini Displayport with an mDP-HDMI adapter, and run the second one with a DisplayLink adapter. DisplayLink is essentially basic graphics card connected to your computer via USB. Because of the limitations of USB, it's not a high performance solution - don't expect to do any gaming on it. This does allow you to use your existing monitors, and at relatively modest expense. You may also want to check the current status of Lion support, as I remember seeing some posts about bugs relating to DisplayLink adapters - they may have been fixed by now however.

The easiest way would be to set the screens to be mirrored again. There is a simple shortcut for this: Cmd+F1 (or Fn+Cmd+F1).

From there, you can open System Prefrences, go to the Arrangement-tab and un-mirror them if you need to, and set the external screen to be the main on (by dragging the white top-strip of the "screen" to the other screen).

Another, maybe better approach, is to use the keyboard shortcut Option⌥+F2, which will directly open System Prefrences to the Displays-board, which will show up on both screens with the option to Gather Windows.