We consider cooperative communications between a source and a destination node with the help of two relays. In order to overcome the half-duplex constraint, the amplify-and-forward two-path relaying protocol is used. We adopt orthogonal frequency-division multiplexing modulation to combat frequency- selective channels and asynchronous reception at the destination node. In this paper, we develop a coherent receiver architecture suitable for unknown block fading channels. A factor graph representing the joint a posteriori probability of the coded symbols and the channels in the frequency domain is introduced. Then, we derive a Bayesian inference algorithm based on message-passing over the factor graph. The resulting iterative receiver maintains low-complexity, based on the interaction between an off-the-shelf soft-input soft-output decoder and a newly introduced per-subcarrier processor for two-path relaying channel estimation and symbol detection. Simulation results show that the proposed solution maintains the full diversity order, even with a limited number of training blocks