Depends on the toddler seat and the double pram you are thinking about. Many prams that take toddler seats are more prone to tipping, but many twin prams are difficult to fit in the car or flimsy. We are going to buy a toddler seat that clips onto the side of the pram and has it's own wheel for support, but I will also have the sling for when Em wants to go to sleep I can put the baby in the sling and put Em in the pram and put the capsule on her seat while it is empty.

Many toddler seats dont recline so it might be worth borrowing a friend's twin for a while and a pram with a toddler seat and see what suits your needs better.