I bought the Baby Trend sit and stand after my younger son was born - he and my daughter are 28 months apart. This was a great compromise since my daughter thought she was too big for a stroller, but we knew better.
As you are searching - Baby Trend does (or did) make a version that had a second seat that was removable and the sit/stand bench was underneath (I think she could stand on the platform, but not sit on the bench with the seat in - but it's been a couple of years so I don't remember for sure). We opted for this model since my daughter was still pretty young and wanted the sit/stand option for her to grow into. This model also has a variety of bars to latch on different "bucket" carseats. It gives you the directions to connect your brand and model of infant seat and it pops right on the front. There were times I wished it had snapped on the back instead - my daughter would get a little frustrated sitting behind the baby, but it wasn't that big of a deal in the big picture. The flexibility of this was very nice though as most "systems" only properly work with their own brand of infant carrier.
For our purposes, it suited us very well. I did not use it all the time, I had a Jeep jogging stroller for my son (LOVED IT!!! and I believe they do make a double-stroller as well) for every day excursions when my daughter could walk along (on more than one occasion she did wind up in the basket under the Jeep stroller).
Now...the downside for the Baby Trend...it is VERY heavy (thus the reason I used the single stroller when possible). It is not terribly hard to unfold, but it's big and kind of awkward. It does not have a very tight turning radius but it pushes very smoothly and we found it to be very durable. All of that said, I did not find any others that were much lighter or could turn on a dime. I researched for months before buying the Baby Trend - it suited our needs the best and I would make the same choice again - it was truly the most flexible of anything else I found and I believe we got more use out of it for a longer time than we would have a regular double-stroller because my daughter had the option to stand. She is now 7 and this will be the first summer we won't use it for amusement parks, etc...
Just my two cents...H.