Dictionaries can’t be sorted as such, but you can sort their contents:
sorted(a_dict.items(), key=lambda (k, (v1, v2)): v2)
sorted(a_dict.items(), key=lambda item: item[1][1]) # Python 3
You can put the results into a collections.OrderedDict (since 2.7):
OrderedDict(sorted(a_dict.items(), key=lambda (k, (v1, v2)): v2))
OrderedDict(sorted(a_dict.items(), key=lambda item: item[1][1]) # Python 3