I have looked after many babies and have used many pushchairs/ double buggies over the years. It is indeed a nightmare choosing a double buggy. Besides which one to get, i had to consider whether it would fit inside my buggy storage box which already holds a few other buggys of all different shapes, sizes and for different purposes. Also, it had to fit in different cars!
I finally came across these Prince Lionheart stroller connectors which are fantastic because i already have 2 strollers to connect! Which means i do not have to get another double buggy and 2 strollers can definitely fit in my boot! and if necessary, they could go on the floor at the back of a smaller car.
It does require the strollers to be fairly similar though. The frame has to match, otherwise you get a crooked 'double buggy'. I've tried connecting my graco to the chicco stroller and it just wouldn't work!