Yes, a dummy plug is to simulate a 2nd monitor. As I have only 1 monitor I must tu use a dummy plug in the second card. Here you have to make easyly a dummy plug only with the dvi-vga adaptor and some resistors. http://www.geeks3d.com/20091230/vga-hack-how-to-make-a-vga-dummy-plug/ There you have the electronic scheme for the resistors.
I see. Thanks for the info. Time to see if I can find some 50 - 150 ohm resistors.