I have an emmaljunga with toddler seat that I use all the time. My two are 19mths apart, and my eldest is nearly 3 and I am just thinking she is getting to big for it. Sorry, not planning on selling it though as I still hope to use it as a single for the baby. (You wouldn't want mine anyway, as I got it second hand and we go out twice a day everyday for relatively long walks and it is starting to keel over from the heavy wear!) I have found it fantastic, but it esp. suits our circumstance (London, no car). It is easy to push despite having 60 pounds + in weight in it (kids and shopping) because it is well designed with the main weight right over the centre of gravity. The suspension and fixed wheels took some getting used to, but now I prefer it. My one had a bassinet bit that the toddler seat could go on top of, which was lovely for a newborn, and then we put the other proper seat on, with the toddler seat on top of that. I don't know about different models, but the handle of my one is quite low, which might be a problem for you. I am short, so it works. I know the new ones do have adjustable handles. As for buying them, I got my used one via the noticeboard on Mumsnet, though there were also some on ebay. I asked for plenty of pictures. There are a few shops in London that stock new ones, but they are quite pricey. When I was looking it seemed that emmaljunga was the only company that had the toddler seat arrangement, but when I went into a shop the other day I noticed that there were a few more companies that were doing pushchairs with toddler seats. These were more the pushchair type, rather than the big perambulator style. I can't remember the names of the companies, but if you like I could check it out and post again. If you are in London, you are more than welcome to test drive mine (I can even supply the kids!)
When I was doing my search, I put 'toddler seat' into Google and that came up with a few options. Anyway, good luck.