dict2 = {'key1': ['value14', 'value15'], 'key2': ['value24', 'value25']}
dict3 = {}
for key in (dict1.keys() | list(dict2.keys())):
    if key in dict1:
        dict3.setdefault(key, []).extend(dict1[key])
if key in dict2:
    dict3.setdefault(key, []).extend(dict2[key])
print(dict3)
Combining 2 dictionaries with common key