In this paper, we study the power allocation problem at the relay nodes for two-hop F/TDMA networks with multiple sources and destinations. Considering the sum capacity as the performance metric, we solve the problem of optimally allocating the total power of each relay node between the transmissions it is assisting. We consider regenerative decode-and-forward (RDF), nonregenerative decode-and-forward (NDF), amplify-and-forward (AF) and compress-and-forward (CF) at the relay nodes. We observe that the optimum power allocation for the RDF and NDF cases are modified water-filling solutions. In RDF, the optimum power allocation considers both the direct links of the users and the relay to destination links, whereas the optimal power allocation for the NDF relaying considers only the relay to destination links. We also observe that relay nodes employing AF or CF may provide higher sum capacities than relay nodes employing DF techniques when sufficient power is available at the relay nodes. Motivated by the optimum power allocation identified for each case, we provide insights to relay selection strategies for relay assisted F/TDMA networks.