Shinzon heard everything that Ruekana and Mack said due to his vampiric hearing, even though they were whispering.  When Rue motioned to him when she mentioned the soul mate he blushes slightly.  And then when Mack spoke to him, he nods and says to her, "I know the feeling."  And then when she mentioned pulling a little number he says to her, "What do you mean by that?"