Hi Andre,
I suggest using the Matrox MXO. Connects to the second screen DVI and USB and outputs either HD-SDI, SD-SDI, Components, Y/C or Composite (FBAS).
Though I would not recommend Composite for On Screen Presentations - quality is very poor - but that is that video standard not the hardware...
But if you use a Blackmagic Decklink instaed of the LFG you could go for HD-SDI to get the best quality.
So you can insert a Keynote presentation running on a second computer into a Catalyst layer.
The SDI setup may cost you some frames of delay - but who cares when running a presentation...
Olli
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Oliver Ranft, Aachen, Germany