The City Select 2 is a special model mostly made for an older and younger sibling. It will definitely not accept two Cloud Z car seats, as these are quite large actually, and this one doesn't even seem to be accepting two car seats in general, not even the smaller ones.
However, the other iterations - the City Select and the City Select Lux will take on two carrycots, and also 2 car seats - I cannot guarantee the Cloud Z compatibility though. They are compatible, there are adapters, just not enough space, I'm afraid. I would go try it out in a store or go for shorter car seats like the Maxi-Cosi Pebble Plus, or maybe, maybeee a Joie i-Level which is also reclinable.
The Gazelle is actually quite large, so not sure if it still can go as a lightweight model that folds compactly - it is more on the bulky side, to be honest. It should accept two Cloud Zs, though.
I personally would recommend a side by side with cots or soft cots, and I would go for smaller car seats if you need a lighter, more compact stroller for twins. Something like the City Mini Double or the Valco Snap Duo.