Generally productions are split, each half dedicated to a different performance, and doing things this way demands an extra level of specificity when it comes to both cinematography and blocking.
As complicated as the normal filmmaking process is, making a feature this way only increases the complexity, and as I recently learned talking with Seth Rogen, it makes what would otherwise be simple scenes into sincere challenges.
When a movie makes the choice to feature a single actor in two prominent roles that regularly interact, certain standard practices on set go out the window as special techniques have to be applied.