PMP8921: Why do we need 2 boost converters in this reference design ?

Hi, 

I am looking at the reference design at . I am not able to understand why we need two boost convertors in the above design. TPS43061   and  TPS63061  both can be used as boost convertor. So why not use one single chip? Sorry for a basic question , but I am a newbie in this. 

Thanks,

Ankit