I don't disagree, and I know I liked that idea early on, but the more I sit with it anything like that is quite clunky. And I know most people would just call it what they want.But the Six Flags part would be just like the Cedar Fair part is for current CF parks— there, but not in the name.
For people my age, Great Adventure is Great Adventure, not Six Flags. For younger people it’s always been Six Flags since Time Warner spent a lot of money rebranding the parks in the 1990’s. Basically they just have to unbrand the parks back to their original names, all with the Six Flags brand de-emphasized.
I think the better move early should be to make a logo for this new venture that blends the identities of both. I think the 'coaster' Cedar Fair logo with a flag off each peak and valley would do, then use the old Cedar Fair font for the Six Flags name. I'm not savvy enough with AI or photoshop to make this, but I think if you did that, just named the parks with a unifying phrase (Like "Great" or something) and use that logo it would work nicely.