We study the performance properties of combined multiple heterogeneous networks. We consider two nodes: node1 and node2, with multiple heterogeneous physical networks connecting them. Each network has some properties such as bandwidth, latency, and packet loss ratio. We develop an analytical model to find the peak performance, or upper bound, of combined networks for the transfer of a message of a given size in parallel. The model is based on the existence of an optimal load balancing mechanism for parallel transfer that fully utilizes the existing networks. The paper provides a framework to find the peak performance parameters of message transfer over combined heterogenous networks as well as to measure the actual utilization and efficiency of parallel message transfer implementations.