Why Should Atomic Blonde Come First?
With an Atomic Blonde sequel, Charlize Theron would have a chance to establish a franchise-leading character. This is significant, as there are a number of projects in the works featuring action-oriented female leads but many are tied in to other franchises. For instance, Ballerina is moving closer to production but is likely to be a part of the John Wick universe. Even with Furiosa, it could potentially grow into its own but would still originate as a spinoff of the Mad Max franchise. Atomic Blonde doesn't have any other connections, however, so it would be free to build a franchise of its own from the ground up.
There's also the matter of which of the two characters fans would associate Charlize Theron with the most. If she became better known as Lorraine, it likely wouldn't hurt Furiosa too much; the character's already been established as part of a larger franchise, and the Mad Max connections would be enough of a draw to bring in some of the franchise's fans even if Max doesn't appear or only has a cameo. If she becomes better known as Furiosa, though, the Atomic Blonde franchise could suffer because some fans might not be interested in seeing "just another action movie" with Theron. It's similar to the success that Mark Hamill had as Luke Skywalker which was followed by years of struggling to get significant screen roles; he was too well known as one character, and studios didn't want to risk their productions being overshadowed by that fame.
Going with Atomic Blonde 2 before Furiosa would also help to establish the new franchise while the first film is still fresh on people's minds. Holding off too long will cause people to start forgetting why they loved the original, and eventually they'll start to wonder "Why are they making a sequel to this now?" You can already see this sort of thing with James Cameron's Avatar sequels, with many wondering if Cameron waited too long to get started on the follow-up. While we won't know until the next film is released, the significant delay may end up hurting the success of Avatar 2. Likewise, waiting too long and letting Theron move on with other projects could damage Atomic Blonde as a brand.
Will It Happen?
Even without considering any of the above, it's still entirely possible that Atomic Blonde 2 will happen before a Furiosa spinoff. Part of the reason for this is that the Furiosa spinoff isn't a sure thing. While fans, Charlize Theron, and even George Miller would like to see it happen, the project has gone from sounding like a sure thing to sounding like something they'd love to do if it worked out. It's possible that Furiosa will never happen, and if it does it might still be several years out. This gives time for Atomic Blonde to strike while the iron's hot.
Of course, an Atomic Blonde sequel hasn't been announced yet, but given the generally positive response that fans and critics had to the film, it would be odd for the studio to pass up the opportunity. Part of it will depend on Theron's availability, of course, since she is now involved in multiple franchises and other projects. If it's just a matter of scheduling, however, it can likely be worked out. Now they just need to make sure that they pull a John Wick and craft a sequel that's every bit as enjoyable as the original film.
Atomic Blonde is in theaters now. Mad Max: Fury Road is now available on Blu-ray and digital.