Hello!
I had an almost two year old and a newborn and a chicco carseat and thought about getting some sort of sit and stand or something to put the newborn seat in. I ended up getting a side by side McClaren stroller instead (off craigslist). The stroller let the individual seats recline, so my daughter could be in it, out of her carseat. When she was little, I took the single stroller and put her in my ergo carrier. When she got a little bigger, I put her and my son in the double stroller. A sit and stand would not work for him, because he is a runner! There is no way to really tie the toddler in well enough to keep my son in. Also, sitting on the sit and stand for the older kid does not look like much fun if you are tired - its not a really comfortable perch. If we were, say, at the zoo, and he was tired, he couldnt really rest/nap on it.
Anyway - a twin umbrella type stroller worked for us, and my reasoning was, ultimately, that she would only be in her carier 10-12 months, and in that time other options would work. A few times my daughter did end up being woken up in the transfer, but she is pretty easy going so it worked out ok.
Gould luck!