I bought a used Baby Trend Sit-N-Stand when I had my third. At that time I had a 3 1/2 year old and almost 2 year old.
It was seriously the BEST $50 I've spent on babygrear.
I still use it for my 3 and (nearly 5) year olds.
The one I bought was similar to this:
You could click the carseat into the front when baby was small enough to be in the carseat all the time.
I would click the baby in front, "sit" my two year old on the seat and "stand" my 3 year old on the platform and push them all in and out of preschool, the library, etc.
I still use it at the zoo, and on walks to and from my sons school.
At two, my kids were able to be on the seat and stand without it being a problem.