In "Mario & Luigi: Partners in Time", Mario & Co. fight the two Princesses Shroob one at time, why?

In Mario & Luigi: Partners in Time, why does Princess Shroob (young) not force Princess Peach Toadstool to deliver the sixth (center) piece of the Cobalt Star, towards the end of the game, to summon her elder sister?

1. Why did she think she were going to defeat the four Italian heroes alone?
2. Maybe she didn’t know that Peach had the sixth piece of the Cobalt Star with her?
3. Or was it a choice of the creators of the game not to complicate the penultimate final fight (last final fight in which Mario & Co. can attack)?