Digi is a much faster grower. They shouldn't grow into each other and the digi should grow around the acro or over the acro, eventually shading it.
They will sting each other, obviously, but there should be room to grow. In a boxing match, acropora species generally have a stronger sting that montipora species. However, with room to work it out, it should alter growth patterns.
Only time will really tell....