Check this article out, it's an interview with Trump.
In it, he states that he anticipated Rebecca being hired, too, and seemed to want it that way, but since Randal was the winning Apprentice Trump allowed him to have a say as a respect thing. Hiring two people was meant to be the twist; asking Randal whether to hire Rebecca was just meant to be a formality - a way to show that Randal was now a decision making member of the Trump team.
Also, what does everyone make of Rebecca being hired by Yahoo? If I were her, I would want to stay as far away from that company as possible- what sort of corporation throws a charity event and then doesn't allow the organizer to actually collect money? They put her in a terrible position, and it's almost depressing thinking about Rebecca in a situation where she has to work under those horrible, mean women again.