K.G.
I agree to have the older child "help" as much as possible. They like being big! I always took my kids grocery shopping with me. It really wasn't all that big of a deal after I worked out a system. I would put the toddler in the cart seat and wear the infant in a sling or Mei Tei (sp?). Getting the kids in and out of the car took a bit of time to figure out. What worked for me was to have the toddler behind the driver seat, and the baby behind the passenger seat. To get them out I would unbuckle the toddler first, close thier door, go around the car and get the infant out. While I was getting the infant out the toddler would be crawling out thier seat and over to the infants side of the car and get them both out the same door. For putting them back in the car it was just the reverse of getting them out. Put the toddler in on the babies side, let her crawl into her seat, them go buckle her in.
Something that I learned very quickly was that most of the time the toddlers needs were much more urgent that the babies. It was much better to let the baby cry for a couple of min while I met the toddlers needs so I could sit and nurse the baby in peace.
I would also suggest that you invest in a good sling or babywearing carrier. Then you will be able to hold the baby and still have 2 hands for your older child!
Blessings, K.