I am a member of TAMBA, Twins & Multiple Birth Association, which seems to be worth it.
I have found useful stuff from various twins websites including:
on line there is www.twinsuk.co.uk/
and www.twinsclub.co.uk/
and www.twinsonline.org.uk/
The one with the best name is MOST - Mothers of Super Twins
For those who like their websites to have an Amercian accent there is the National Organistation of Mothers of Twins
I have a job writing about Kid's Products. You can find a list of all my articles here at Suite101, where I have written quite a number of pages on twin pushchairs and travel systems for twins. Yes, they do exist and there is a choice. Actually, I have also reseached buggies for quads and 6 children.