I'm sorry, but there is simply no way that Cowell could not hear her, or had no care to hear her, not at the precise moment that she held the casting vote to determine if one of his acts goes home.
He suddenly developed selective hearing because she was seeking to confirm what he himself has said several times live on air - that it is a singing competition. He knew what she was getting at, and that she would use that as her criteria for sending the twins home (as if she was going to vote off her boss's act anyway).
While he's happy with that result, he has also realised that though they may not be singers the twins are potentially his biggest money maker in a bit of a dry season, so he didn't wish to be seen to be party to their downfall when another judge checked with her boss and show creator what the criteria should be. Instead he wants to parade them around as his discovery. That's also why he didn't send them home sooner when he got the chance.
So he let Dannii be the fall girl. Then if any backlash ensued, as it has, she gets it, with calls for her sacking, while he remains squeaky clean and in the $imoney as always.