TTC, our first two are 16 months apart, so we got the Babytrend Sit N' Stand with an optional back seat. So for a long time we just had DD in the seat in the back and DS in his car seat in the front, facing us. Once he got older, we had them both in their seats, and now that DD is older, we have the back seat turned into the sit n' stand bench so she can get in and out when she wants to.
When this baby comes, we'll go back to the same set-up, but with DS in the back and new baby in the car seat. DD will either walk, or if DH is with me, he can push her in an umbrella stroller.