When I had my second my eldest was only 16 months old, so I didn't trust him not to poke the baby etc. I had my toddler on the passenger side and put the baby in a capsule behind the drivers seat. My reasoning for this was that I could sit the capsule on the ground or have the baby in the pram on the footpath whilst getting my toddler into his seat, then I could deal with that baby. I really didn't trust my eldest not to want to run around to his door to try and get into the car (he could be very independant).
It worked for us. Now I have 3 kids so there is no room for anyone in the car anyway.