R.C.
Hi there,
I have the Joovy sit and stand (not sure if it is the ultra-light). I loved the idea of the sit and stand because my two year old is very energetic and the option of sitting v. standing was very appealing. We have had our nieces (the youngest was about 18 months) strapped into the back with no troubles. Just make sure he is strapped b/c they can fall through without it.
Just want to give you a couple warnings about this stroller: it is not very maneuverable, in fact, it is quite difficult to get it to turn. Although it is lightweight, which is great for lifting, it is not easy to turn at all. I have a heck of a time with it and I don't have any problems with my wrists or arms. Also, it has very little room for the toddler on back and virtually no storage room in the basket underneath.
That said, it has been a decent stroller overall and I do still like the sit and stand option. However, given the difficulty in maneuverability, I would not buy it again, nor would I recommend it. I would definitely go with the Maclaran side-by-side instead. More expensive, but so very easy to maneuver, which, to me, makes it worth every penny. Good luck!