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When running two RHD chips on one cable, the voltage drop over the cable resistance is larger. In extreme cases, such as when using a very long thin wire tether, the VDD can drop close or below to the required 3.2V. Use the on-chip voltage level reader to verify that the chips are receiving adequate voltage. If this is an issue, you can either choose a lower resistance VCC/GND wire pair, and/or slightly increase the supply voltage on your acquisition board by replacing the resistor on the 3.5V headstage power regulator.
Flexible PCB dual-headstage adapter
Intan now offers an equivalent flex-pcb based Y-adapter (link) and RHD2000 Dual Headstage Adapters) and made all schematics available.