when i use ADM2682E between two nodes of which the distance is more than 100m, do i need to connect the two GND2 of ADM2682E to provide reference? Or just connect the bus pins of two ADM2682E chips with termination resister is ok ?
Isolated RS-485 transceivers do not need their GND2 nodes connected together in order to communicate. Both transceiver's GND2 are floating with respect to Earth, and will track each other through the input resistance of the transmitter and receiver pins. Connecting up the bus pins will be sufficient.
In noisy environments, as part of EMC measures against external noise, it is often recommended to connect the grounds together.