In conventional two-tiered Wireless Sensor Networks (WSN), sensors in each cluster transmit observed data to a fusion center via an intermediate supernode. This structure is vulnerable to supernode failure. A double supernode system model with a new coding scheme is proposed to monitor a binary data source. A Distributed Joint Source Channel Code (D-JSCC) is proposed for sensors inside a cluster that provides two advantages of low complexity transmitters and scalability to a large number of sensors. In order to setup a robust communication channel from sensors to the data fusion center, Distributed Space-Time Block Coding (D-STBC) is employed at two supernodes prior to relaying that results in additional diversity gain. DeModulate and Forward (DMF) relaying mode is chosen to enable packet reformatting at the supernodes, which is not possible in widely used Amplify and Forward (AF) mode. The optimum power allocation for the two-hop multiple DMF relaying is calculated to minimize the system Bit Error Rate (BER). An upper bound is derived for the system end-to-end BER by analyzing a basic decoder operation over the system model. The simulation results validate this upper bound and also demonstrate considerable improvement in the system BER for the proposed coding scheme.