My understanding has been they are supposed to fill the buckets only, but it's a YMMV situation. I've seen more people having better luck having it separately with the fancy buckets, that's for sure. I'm guessing maybe they're a bigger pain to fill and CMs are more happy to bend the rules? Or maybe it's just a rule that some CMs decided to implement?
I did have one CM (not AP buckets) say she was doing my mom and me favor letting us each do a separate transaction. She said she was supposed to send us to the back of the line (I had asked if we could buy our cauldrons [green] together to speed her line or if due to purchasing rules they had to be separate; I asked only because some CMs did let us just do one transaction for both of us to buy a special item to speed the line up). She had clearly interpreted the "one per person" as "one per group". Or maybe she thought I was a teenager and mommy was buying for me anyways
So maybe they can be on the side no problem now and I've just been hearing made up/misinterpreted rules? Who knows. It's Disney. It feels like there's a different set of fine print every day based on who one talks to
They give you the standard box. The standard buckets were about a box and a half fill I think? And I think the fancy buckets hold roughly the same. So if true, then getting it on the side one is technically getting less. But those fancy buckets are a pain to clean so there is a definite benefit to having the popcorn on the side.