dict1 = {'key1': ['value11', 'value12', 'value13'], 'key2': ['value21',
    'value22', 'value23']}
dict2 = {'key1': ['value14', 'value15'], 'key2': ['value24', 'value25']}
dict3 = {}
for key in set().union(dict1, dict2):
    if key in dict1:
        dict3.setdefault(key, []).extend(dict1[key])
if key in dict2:
    dict3.setdefault(key, []).extend(dict2[key])
print(dict3)
Объединение 2 словарей с общим ключом