The two truths that are told are the ones where he is heralded as Thanes of Cawdor and Glamis...the "imperial theme" is the ultimate promise of the throne that Duncan currently holds. Macbeth has just come from battle (and from speaking with the weird sisters). If he doubted the word of the witches before, he does not doubt now. In his choice of the words "imperial theme". he reveals his ambition: he wants the throne. The question is...would he have wanted it if the witches hadn't told him it was possible? Would he have even thought of breaking his faith with Duncan had he not seen the "truth" in what the weird sisters said? Unlike Banquo, who mistrusts appearances of truth (Banquo says often the instruments of darkness will seem to be offering something good and true only to win a person to harm), Macbeth sees what he wants to see and doesn't waste any time questioning whether or not the "truth" he sees is right or good.