Let C be a triangulated category with shift functor [1] and R be a rigid subcategory of. We are concerned with almost complete two-term weak R[1]-cluster tilting subcategories which can be completed to a two-term weak R[1]-cluster tilting subcategories by one indecomposable object. Our main result shows that such an almost complete two-term weak R[1]-cluster tilting subcategory has exactly two complements. Moreover, we offer explicit constructions for these two complements via mutations. As an application, our main results can be applied to the functorial version of τ-tilting theory.