Well, the naive solution might be something like this:
a, b = dict(), dict()
a["a_one"], a["a_two"], a["a_three"] = "a1", "a2", "a3"
b["b_one"], b["b_two"], b["b_three"] = "b1", "b2", "b3"
for k in a.keys():
if b.has_key(k): continue
else: b[k] = a[k]
After the loop terminates, b has all of a's items, except for the ones where the keys collided. You need to decide for yourself what should happen on key collision - should you raise an Exception? Overwrite the value with a's value? Let b's value stand? Do something else? The answer to these questions is situation-specific.