JB's spot on about the complicated dance involving multiple kids and going out in public. I developed a few strategies. First and foremost I taught my oldest to hold on to my trouser leg whenever I let go of his hand. Next our car is large enough I could unbuckle my oldest and he could climb down from his seat, standing in the backseat while I dealt with my little one. Or I could have him climb into the car and across to his seat while I got my little one buckled in. That was a huge help so I didn't have to worry about my big guy standing around in a parking lot. Finally I always took a less is more approach to outings. I never packed too much in a small carry bag - one diaper per kid and wipes mostly. Everything else (change of clothes, extra diapers, wipes, roll of paper towels, etc.) lived in the trunk. No point in dragging around more than needed and if it comes down to a diaper or clothing disaster, chances were heading to the car was the wisest choice anyway.
Mine were close in age (less than 2 years) so I had to constantly remind myself how little my big one was. Honestly, though, no matter their age everyone struggles with the transition so deep breaths all around. I wish I had done more one on one outings with my oldest before but we managed to get a few trips in. Also I wish I had put my oldest in some kind of care a few days a week so I could really focus on my baby. I did a disservice to all of us in the beginning trying to juggle everyone and not being able to just focus on one at a time. I needed help and should have sought it out sooner.
I deliberately put my baby after my oldest many times. I never wanted my oldest to feel like a second class citizen as a result of the new baby. I can remember swaddling my screaming baby and plopping him down for a few minutes while I got my oldest milk or a snack or whatever. The whole time I talked all of us through the experience. "Baby, you are fine; babies cry, sometimes a lot. Big brother, let's get you what you need and let's talk to Baby so he knows he can hear him. Big brother, what do you think Baby needs?" etc. etc. etc. Even know my oldest is very calm when my little guy pitches a fit. He just looks at me and says 'babies cry, right, momma?"
For us as working parents finding one on one time with everyone is tough. We put our littlest down first and spend time with our oldest. Then he goes to bed and it's time for us as a couple or as individuals. In general for me the single biggest culture shock was the stark reality somebody always needs something. It can run you ragged at times; you get one settled only to have to deal with the other and then it starts all over again when done.
P.S. Beware the oldest trying to help. For example I turned my back for a moment and that was all it took for my oldest to pick up my baby. BAM! He hit the hardwood floors and chaos ensued. I had both of them shrieking and it was madness to calm down. Not my finest moment for sure but a lesson learned.
P.P.S. Shopping carts and multiple kids and your groceries/purchases. This one was always a pain for me and I don't have any words of wisdom.