Within the energy sector, two of the most tightly coupled systems are natural gas and electric power. The recent advent of cheap gas extraction technologies have only driven these systems more tightly together. Despite their interconnections, in many areas of the world these systems are operated and managed in isolation. This separation is due to a number of reasons and challenges, ranging from technological (problems involving connected systems are difficult to solve) to political and commercial (prevention of monopolies, lack of communication, market forces, etc.). However, this separation can lead to a number of undesirable outcomes, such as what the northeastern United States experienced during the winter of 2013/2014. In this paper, we develop approaches to address the technological reasons for separation. We consider the problem of expanding and designing coupled natural gas and electric power systems to meet increased coincident demand on both systems. Our approach utilizes recent advances in convex modeling of gas and power systems to develop a computationally tractable optimization formulation.