How to enable Do Not Disturb for 1 SIM only in a Dual SIM phone?
Method one
Disbable the notification tone and the ringtone for the simcard that you do not want to use. Just go yo sounds and set it to none for sim 1 or two.

Method 2:
This is a tricky one since there is no default method that i can find to do this, but if you have used tasker before and your phone is rooted then there should be a plugin or a method to do this. 

Tasker on Playstore

Goodluck mate