You should ask even stronger- Rav Yitzchak himself just said those words when discussing the halacha, so Rav Nachman should have asked him about that! However the Machane Chayim suggests that there are three different ways to say Rachav Rachav 1) When one has no thoughts about women 2) when one wants a beautiful woman for his wife 3) when one lusts after a beautiful woman in front of him. Chazal say (Sotah 8) that the Yetzer can only have control on what a person sees and in this case there is no woman there so Rav Nachman thought it was ok to say. However He thought Rav Yitzchak was saying that if a person even thinks of a woman without seeing her he will spill seed and Rav Yitzchak answered that this is only if he actually saw her once as then the Yetzer Hara can have control on him.

It's also possible that Rav Nachman was saying that he had said it in the past and nothing happened to him or that he was pointing out to Rav Yitzchak that he (Rav Yitzchak) himself had said it and therefore it was not a problem. (Interestingly, in one Gemara-Megilla it says that Rav Nachman said "I said Rachav Rachav" and in the other Gemara it says that rav Nachman said "I said it." However I noted in a number of manuscripts of Megillah that Rav Nachman says "I said it" without mentioning the name)

Yoel Domb