I do wonder why a Franxx can't be driven by one person and the dummy component in place of the other, even though I can easily imagine Dr Frannx settling on the live pair design for the personal philosophical reasons he outlines in episode 1.
Having said that, this is a convention going back at least as far as Five Star Stories and its Headliner/Fatima pairs, and maybe we'll see some uncrewed units deployed later on that fill the venerable role of automated warmachine bogeyman, in the vein of Mobile Dolls and the MP Evangelion series.
There's going to be a "don't do that or you'll go blind!" joke in there somewhere. Either that or because this has Gainax folks the side-effects of operating a Franxx alone is similar to breaking the 400% synchronization in Evangelion, you become one with the machine...possibly for good.