What type of VFD's are you using? Many "Estop" inputs do not actually e-stop the drive. Most fancy drives that have this capability, refer to the input as a "safe off" input, and requires a dedicated loop, with current and pulse monitoring.
Most consumer level drives do not have this functionality, and therefore require a separate method of e-stop. The most common is a contactor which removes main power from the circuit. If you were to wire the coil of this circuit through your normally closed switch, you could feed both drives from it, still have your extra pole on the switch, and have a better Estop system.
-Cody