Callum,
Thanks for the response to Keith’s question. However, your answer is not entirely true. According to page 17 of the Intensity Pro manual On-Air 2.0 can be used with 2 Intensity Pro cards (as long as HDMI is used on both cards).
https://www.blackmagic-design.com/downloads/manuals/IntensityManual.pdf
To make things more complex, this statement is not true either. The Intensity Pro in the top-most slot can be configured using the Blackmagic Preferences to use an Analog input while any other cards will default to HDMI. Due to limitations in the driver other Pro cards cannot be used to capture Analog sources.
As 1 of many people out there pondering the possibility of purchasing 3 Intensity cards to do live HDMI (compressed) switching, I think an official (and tested) stance on this from Blackmagic is in order. With all the confusion and questions regarding multiple Intensity cards in one Mac Pro, it is up to Blackmagic to take the reigns and clear things up publicly.
Thanks,
Mark