I am basically deciding same stuff for my machine.

So i wired Estop to the servo drives not to the board. The board just enables the drives. So I hit the Estop and the drives stop instantly, they emit alarm signal that disables the board and puts all in reset condition. I reset from mach3 and board resets drives and enables them/ 2 different pins that go to relays/

What i am saying is that is ok for the board to have second circuit connected to the estop button, but its a must to stop the servos, so thats why servos usually have Estop and limit switch inputs, to react faster than boards.