Near the corner, if the stem walls are connected, the walls will be restrained from movement and will therefore be subjected to at-rest earth pressure. With the walls tied together, overturning won't be a problem, but you will have to structurally design the connected sections for the loading. Otherwise, there's the alternate route where the stem walls are isolated so that they function independently. That's how we handle the wingwalls on concrete box culverts over 8ft tall. Under 8ft, we detail to pour the end wall and the wingwalls monolithically.