Why does the carbonyl group in an acid anhydride have two stretching frequencies?
The two observed C=O frequencies are due to the symmetric and asymmetric stretching modes of the anhydride.


  
  Source: Introduction to Spectroscopy, Pavia and Lampman


You can see that the lower frequency symmetric stretch occurs where both C=O bonds are lengthening and shortening in tandem, whilst the higher frequency asymmetric stretch occurs when one C=O group is lengthening as the other is shortening.  You can see an example of this kind of spectral pattern in the example below. 


  
  Source: Introduction to Spectroscopy, Pavia and Lampman


This explains why you get two frequencies for C=O bond in acid anhydrides, whilst ketones/aldehydes etc. only give a single IR frequency for the C=O bond.