A = {'a':1, 'b':10, 'c':2}
B = {'b':3, 'c':4, 'd':10}

{'a':1, 'b': [10, 3], 'c':[2, 4], 'd':10}

{k: filter(None, [A.get(k), B.get(k)]) for k in A.viewkeys() | B}

>>> A = {'a':1, 'b':10, 'c':2}
>>> B = {'b':3, 'c':4, 'd':10}
>>> {k: filter(None, [A.get(k), B.get(k)]) for k in A.viewkeys() | B}
{'a': [1], 'c': [2, 4], 'b': [10, 3], 'd': [10]}

dict({k: [A[k], B[k]] for k in A.viewkeys() & B},
     **{k: A.get(k, B.get(k)) for k in A.viewkeys() ^ B})

>>> dict({k: [A[k], B[k]] for k in A.viewkeys() & B},
...      **{k: A.get(k, B.get(k)) for k in A.viewkeys() ^ B})
{'a': 1, 'c': [2, 4], 'b': [10, 3], 'd': 10}

from collections import defaultdict

A = {'a':1, 'b':10, 'c':2}
B = {'b':3, 'c':4, 'd':10}

result = defaultdict(list)

for d in [A, B]:
    for k, v in d.items():
        result[k].append(v)

defaultdict(set)