In late 2005, I was playing in a chess event where GMs and IMs did not yet know about Rybka. These days, almost all GMs know about Rybka and use it. The new volumes of the Roman's Labs DVDs claim all their analysis is Rybka checked for instance.
GMs do not try to understand plies, NPS etc. When they test engines, they look at the evaluation of a particular position. They know that Rybka is a very strong engine. Some GMs have also mentioned how they like Rybka's stable evaluations. Using more than one engine is not common for top players. Likely that top players use Fritz and Rybka if they use more than one engine.
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