I have newborn behind driver and almost 2 year old behind passenger.
My 2 year old can climb into the car seat but at the moment chooses not to, clibing anywhere else in the car is way more exciting.
I take the baby out and put her in a carrier or pram, and can then get the toddler out onto the footpath.
The toddler is also a runner, and i wouldn't like to lose grip on her hand if i was getting her out on the road. At least on the path, there is longer for her to get to the road, and hopefully i woud grab her again before she reached it.
Also don't have the two seats next to each other cos for now, the toddler would bash or feed the newborn.