def deep_merge_dicts(dict1, dict2):
    merged = dict1.copy()
    for key, value in dict2.items():
        if key in merged and isinstance(merged[key], dict) and isinstance(value, dict):
            merged[key] = deep_merge_dicts(merged[key], value)
        else:
            merged[key] = value
    return merged

# Example Usage:
# dict_a = {'a': 1, 'b': {'c': 2, 'd': 3}}
# dict_b = {'b': {'d': 4, 'e': 5}, 'f': 6}
# merged_dict = deep_merge_dicts(dict_a, dict_b)
# print(merged_dict) # {'a': 1, 'b': {'c': 2, 'd': 4, 'e': 5}, 'f': 6}