For those of you with 2 in car seats..

Kristi • I'm a stay at home mom to a little boy who's 2 years old. I'm currently pregnant with my second who is due 10/17/16.
I have a 2 year old and am due in 20 days. I plan to clean out and rearrange my car to install the 2 car seats. Just wondering does it matter which is on which side or is it just preference? (if preference please share your experience)