If i remember correctly you can do a custom order and get a z2 with a b2 first stage
I just looked through the dealer catalog. I suspect what you said is the easiest way to get a Z2 with a swivel turret. The Atomic website lists the Z2 1st stage as having an option for a swivel turret. After looking at the dealer catalog, I suspect what that means is that you can order a Z2 and the swivel turret parts from a B2 (or whatever) and install the swivel turret to replace the stock, fixed Z2 turret. It doesn't look like there is a way to just order the Z2 1st stage already made up with a swivel.
And with the price of parts and labor to install, it's probably cheaper to just get a B2 1st.
All the 1st and 2nd stages are available to order individually. So, you could order a Z2 2nd stage with any other 1st stage. And vice versa. But, there is definitely a price break on ordering a set. E.g. ordering a Z2 1st and 2nd together, versus an individual Z2 first and individual Z2 second. A Z2 set is $440. Ordering the 2 parts individually, the total would be $600. In other words, even though you can order the 1st and 2nd stages individually to mix and match, you will pay a premium to do so....