i think its easier to have the baby go in the passenger side, so you can pop the seat in very quickly(assuming its one that just pops in the base), then toddler on your side. the reason is so you can buckle in the toddler without needing somewhere to put the baby and make sure to get the baby in right away with the bad weather. then you have 2 hands to unbuckle the older one behind you before walking over and quickly getting the carrier out.
so i walk to the car, put baby in passenger side snapping in the seat, then walk around with toddler buckling them with both hands, and i get in. i found the 2 seconds letting go of my older ones hands while unsnapping(snapping in takes on hand) the car seat from the base was better than having to set the carrier down to buckle the toddler, or walk back and forth.
also, its nice to be able to see the baby.
now when the oldest can buckle themselves, and it does happen, you can switch so you just open the door and they get in to do it themselves. then you go to your side to buckle the youngest. it cuts the time in half.