The most correct electrode layout is as shown below, such that distance ‘a’ is greater than distance ‘b’. The stimulus current will flow between DRVP and DRVN, while the voltage is received by BIP and BIN, so you want BIP to be near DRVP, and BIN near DRVN. You also want the vectors DRVP->DRVN and BIP->BIN to be parallel so that the received voltage is in-phase with the stimulus current.