N.A.
I used to work for Babies R Us in the baby gear (car seat) department and we had to know all about car seats and safety, and I worked with the local car seat safety group that came to our store and did physical car seat safety checks...so I hope my two cents will help!
After your 2nd is born, put that child in the middle, and put your oldest behind you. I know this will be a little bit of a pain for you when you want to get the new baby in the car by having to go to the other side of the car to put him/her in the base. However, keep this in mind. The first safest spot for a child is in the middle (where your baby will go); the second safest is behind the driver (because if you were to potentially get in an accident, it is human instinct to try to get themselves (in the drivers seat) away from harm, therefore, you'd be opting to get your child sitting behind you out of harms way too); 3rd would be behind the passanger (only other option, right?!).
If you can't fit both carseats side-by-side in your car, then put the baby behind the drivers seat and the oldest behind the passenger.
On a side note - we had a rental car for a few weeks when both of my children were really young and there was not enough room to have them sit side by side. So, my husband, being the smart thinking he is, made the kids' car seats straddle the seats. For instance, my youngest was latched between the seat behind me and the middle, and my oldest was latched to the middle and the passenger seat. Sounds strange, but it most definitely worked for such a small car and kept them both in there tight (like any other latch system) and kept them away from the sides of the car. (Glad it wasn't for too long...but it worked...)
Hope this little bit of info helps! :) Good Luck...and Congrats on the soon-to-be new baby! :)