I learned that it is not necessary to have two video cards to display on two devices. My daughter, who uses Macs in teaching, loaned me a DVI-to-VGA adapter, and I'm now looking a different screens on the Macbook and an external monitor. She told me when she runs PowerPoiint the PP program is on the laptop screen and the slides display on the projector connected to this adapter.
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Later: I did some fiddling and learned I needed to set OpenLP to show the projection image at X=1440, the width of the Macbook monitor.
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Lane