This can be done using numpy.core.defchararray.add. Here is an example:
>>> import numpy as np
>>> a1 = np.array(['a', 'b'])
>>> a2 = np.array(['E', 'F'])
>>> np.core.defchararray.add(a1, a2)
array(['aE', 'bF'],
dtype="<U2")
There are other useful string operations available for NumPy data types.