We have a BMW X3 (smallish SUV) and our (40 lb) 3 year old is FF in a True Fit Premiere. Our baby will be in a Snugride 35, which we are installing today.
Right now, we have the TFP on the passenger side. He is independent about getting in (climbs in the car himself, buckles the chest clip, I just have to do the bottom clip).
Trying to figure out if it's better to put them next to each other (so we have the option of me sitting back there if I need to) or put them apart so he can't disturb her as much if she's sleeping, but we lose a seat.
Also-- trying to figure out if there's an option that doesn't involve walking to both sides of the car, since we're in NYC and park on the streets only and it's tricky holding onto 2 kids in busy traffic with cars whizzing by very close to us.
So I thought maybe I'd do what my friend does and put the baby seat on the driver side, then the TFP in the middle? That way he could potentially climb over/under her seat if need be, otherwise I could get him in on the passenger side and just reach over to clip then walk around with baby and get her in. He may protest not having a window.
Speaking from experience, I would not put them side by side. Most 3yos will not leave a baby alone, especially when bored in the car, as you will find out from the screaming/crying you'll hear when said 3yo pokes their sibling in the eye, etc!
As far as loading, I would put the TFP on the driver's side. Then your oldest can climb in the passenger side and over to the TFP. Then you can either reach through and buckle or after you get in sit on your knees on the front seat to turn around and buckle the 3yo. Then infant seat can just snap in on the passenger side.