My customer uses only 2 (1-1) ports though ADuM6402 has 2-2.
How to treat the pins which are not used.
Can they connect GND?
The ADuM6402 inputs that are not used can be connected to GND (or to VDD). The unused outputs should be left unconnected.
If they only need 2 channels they could also use the ADuM6201, which has 1 forward channel and 1 reverse channel.