We develop a method for designing symmetric modifications to linear dynamical systems for the purpose of optimizing ℋ2 performance. For systems with symmetric dynamic matrices this problem is convex. While in the absence of symmetry the design problem is not convex in general, we show that the ℋ2 norm of the symmetric part of the system provides an upper bound on the ℋ2 norm of the original system. We then study the particular case where the modifications are given by a weighted sum of diagonal matrices and develop an efficient customized algorithm for computing the optimal solution. Finally, we illustrate the efficacy of our approach on a combination drug therapy example for HIV treatment.