You now have two grandkids you have had the one in a car seat in the middle back How should you position the new car seat in the back seat you believe you can use all three positions?

You now have two grandkids you have had the one in a car seat in the middle back How should you position the new car seat in the back seat you believe you can use all three positions?
It really depends on the car seats that you have. How old/big is the older child? Is s/he in a booster or car seat?

You should be able to put one seat on either side of the car, with the middle seat empty. Depending on the size of your back seat and the sizes of the car seats, you *might* be able to squeeze someone into that middle seat.

Also, depending on the age/size of the older child, you might want to consider leaving him/her rear-facing for as long as possible (i.e., according to the limits of your particular seat). It's much, much safer for the child.