There will be only one edit tab. When you click on it, an editor will be loaded. Depending on user and system preferences, which editor loads will differ. For users who have no preferences beyond the default, which includes anonymous users, they will get the editor they used last; for logged-in users who've never edited, or logged-out editors who've not edited since their cookies were cleared or expired, they will get the "default" editor.
On "VE primary" wikis (like the Spanish Wikipedia, along with >200 others), the first editor that loads will be the visual editor. As always, first-time editors on opening the visual editor get some educational material welcoming them to the wiki, and about how to use links and references appropriately. Returning editors will get their most recent editor. Returning logged-in editors with a one-time welcome dialog that offers the ability to change their preferences in case they don't like the default settings (e.g. they never want to load the visual editor on clicking 'edit', or they want both tabs). This is hopefully going to go well, but as hope doesn't make a promise we're doing it gradually, starting with one smaller wiki and verifying it works for users there before proceeding.
I hope we'll be able to run the A/B test for the English Wikipedia in the next couple of months, but our first duty is to ensure these changes work well on the bigger wikis by VE edit volume.