A lot of stuff would be pooled between the regions anyway because it was too big for one region to do. In one of the IBA Yearbooks they talk about the 1975 European Cup Final at Hampden Park which was an enormous event, so while STV provided the main coverage there were also OB trucks from ATV, LWT and Tyne Tees at the venue to provide additional resources.
Yep. 1977 IBA Yearbook, page 36-39
There's indeed a picture of STV, Tyne Tees, and ATV OB trucks parked up together at Hampden. Also a link was established to LWT and back, for slo-mo replays using LWT's hideously expensive Video Disk recorder.
The Pope's 1982 visit to the UK, was too big and spread out for either the BBC or ITV (as a network) to cope with alone, and coverage there was pooled between all available BBC and ITV OB trucks