It is possible, but not easy at all. I tried that with the Quinny Hubb and what I did was to climb backward, pulling the stroller up on the back wheels, but you need to be ready for quite some strength needed to do that. It is possible that in the end, the easier way would be to just put at least the older one out of the stroller. This backward pulling would only be possible with the Cybex though, at least when we talk about a setup with a carrycot - later on, with two seats, the Harvey can be pulled up too, but I don't think you can do that with the carrycot and a seat on that one.
About the height, you are right that the back child, which is usually the younger one, is quite low (in the Cybex's case, it's the older, in the front, who'll be lower). Not ideal, of course, but the idea is to save space and have a nimble-ish stroller which is why space is not optimal, perhaps, but practical for maneuvering. I like the Harvey's setup a bit more, even if it's maybe a bit more cramped, but for maneuvering, to have a child in the back makes for a much better center of gravity. Yes, the Gazelle can have that setup, but only parent facing or straight-legger for the older one, which can be done but may be not ideal.
So personally, is I would be walking a lot and even going up stairs, I would maybe think about the Harvey a bit more, myself, even if Gazelle feels a bit more modular. Also, the nice suspension on the Harvey is something I really liked. That being said, both are very nice and not a bad choice at all, so choose with your heart, I don't think you will make a bad choice out of the two. If pushing comfort is the priority, go for the Harvey, and if the space between the seats and modularity are the more appealing feature to you (plus the pulling up the stairs), go for the Gazelle.
Also good to know - a 2 years old may be a bit big for a reversible seat. It would be good to try the strollers out, and perhaps think of something with a bench seat instead, e.g. the Baby Jogger City Select Lux, space-wise.