David
I think it depends on the numbers of pairs and avaliability of sites (not forgetting of course natures supply of food, which can often determine success or failure). I must admit I am not sure of the scientific approach to this question, but look at it this way; if 3 pairs use 3 different facing nests, there is an adequate/abundant supply of food, cover and space for them to fledge and develop into adults, then why not?
It's well known that Blue Tits, out of the breeding season, work hedgerows, trees and other food sources in small to medium size groups/flocks. Are these family groups, or just communal groups? I don't know. Obviously they are not recognised as social species like Sparrows, but that is not to say that occassionally they cannot. Often we hear of more than one nest in what are not neccessarily large gardens, and although they may not be in a "terrace", I would have thought that according to the known law, they are more than close enough to warrant territorial problems. But they survive.
You may have a unique opportunity to actually witness something that not many other people have the chance to do, and I look forward to reading continuing posts of their progress.
We have never had more than one nest at a time in our garden of either Blue or Great Tits, but even between the nests of both types there has never been more than a few yards, and we always have one brood of each.
Perhaps that is the answer.
Hope I have not confused you too much. I new what I wanted to say but seemed to be getting bogged down in trying to get out a reply. Hope it makesd sense!
Regards,