that's fine, except that most (all?) twin wastegate manifolds for skyline 6s I have seen run a balance pipe to connect the two manifolds.
with response, I was talking about the response of the wastegates, not boost response or engine response, and two 40mm gates should be more responsive to changes in pressure that say a single 60mm, but again it would not really be enough to be noticeable.
but sure, twin gates is nice, and if you are running twin big turbos then it's the best option for sure. but for a single turbo I would always just run one big, single gate.