I would have a look at the shortcuts in Navigate ▸ Move Focus To, and in particular the top one. You might also want to change it, in the Keyboard options tab (search for “Supporting Editor”, confusingly). I changed mine to Ctrl+Tab.
This shortcut will rotate you between all major visible views. So with all of these currently visible: Binder → Top/Left Editor → Right/Bottom Editor. But those other shortcuts are not to be understimated. As much as I use Ctrl+Tab, I probably use the others even more, because I do often have the editor split, and am wanting to get straight to something specific. Navigate ▸ Inspect is another good submenu to memorise.
I wouldn’t try using Tab for this (with regards to customisation). That key is generally used for more detailed button and control navigation (even on a Mac, though there it is a deeply buried option). You could Tab all the way into the editor, but it would take about half a dozen to get there (across the header and footer bar buttons). Might as well just Ctrl+Tab between the two, or… whatever that weird default shortcut is that I always change immediately. 