Some moms shared that the new child slept in the parents' bedroom for up to six months. Others shared they moved the big kid into a new big kid room, so baby could have original nursery. Others kept big kid where he was and just moved crib out and nursery items out to a second room to start over.
It all really depends on your home, lifestyle, age of children you already have, etc. to determine how you're going to fit #2 into the mix.
Some get very concerned with moving #1 out of the crib so they have it ready for the second baby. I always caution moms about this, as when you move the first child into a toddler or other bigger bed it typically means that they don't sleep as well as they did previously since it's a change in routine. You will WANT them sleeping when baby comes! So think this one through... if your first is not crawling out or getting out of crib just yet, leave them there... baby will be in your room probably for a few months anyway, which gives you even more time to move your first into a big bed if you need to. Baby also could sleep in a pack n' play if necessary until big kid is out of crib for a few more months. Take your time, it will all work out!
As for car seats: some like baby behind passenger seat to see better in back, some like in back of parent some keep baby in middle with toddler on the side. It depends on your child. I would never have had the baby next to the toddler in the middle because I was afraid he'd toss a toy or book at it accidentally. I had the baby behind driver's side, as it was easier to get baby out that way for me. I made sure I had a rearview mirror in the back with baby so I could see that they were ok. I never needed that with first child, but having two back there, I just wanted to be sure all was all right. I had my bigger boy behind passenger seat. It was easy to pass things back to him also.
It all depends what works for you, that's what I've heard from moms in the group. So do what works, and be patient with the big kid with whatever changes you're making.