MatNanPlus - yes I think you can do that, as on the website they have pics of the carrycot (which uses the same seat frame as the seat unit) on the back and the front seat unit sitting upwards. You have to have both seats facing the same way though, but that wouldn't bother me. The car seat would only be used for the school run, I wouldn't put the baby in it for more than 1/2 hour.
My usage of a double pram will be:
Pram in car, get kids into car, take DD1 to school (have to drive, too far to walk), get pram out, put DD2 and DD3 in it (hence would be easier if baby kept in car seat) and walk 10 mins to school (have to park at a friends, it is mega hard to get near to the school to park), walk back with DD2 and DD3 and get back in car. Maybe in the afternoon I will go for a walk to the shops with DD2 and DD3 and get a few bits in for tea or pick up clothes from our town centre (only 15 min walk away). I don't do any off road walking and it is pavements all the way. Then back to do the school run, the same as the morning.
Who can advise me what the best double would be for this? I have a M&P Pliko which can be used as a single when I only have 1 of them with me so a side by side is an option.
I definitely do not want a Maclaren, I have one a friend gave me and it is awful to push. I suffer with a bad back and shoulder so need something nice to push (I loved my Bugaboo).