Sam,
From what I know Thomas Andrews was Head of the Design Department and Managing Director of Harland and Wolff. It is said that he knew almost everything related to shipbuilding. A worker at Harland and Wolff stated that Andrews could have built a ship himself and sailed her,too.
Although Andrews took a very high position he was very close to his workers and was always ready to make a suggestion if help was needed.
I never heard that John Brown yards called upon Andrews but I think that it might be possible as he was known to be one of the best naval architects.
Bye,
Julia Feldmann