Bilateral Shorted Operators and Parallel Sums
  In this paper we study shorted operators relative to two different subspaces, for bounded operators on infinite dimensional Hilbert spaces. We define two notions of complementability in the sense of Ando for operators, and study the properties of the shorted operators when they can be defined. We use these facts in order to define and study the notions of parallel sum and substraction, in this Hilbertian context. 
Jorge Antezana, Gustavo Corach and Demetrio Stojanoff