Dual SIM sound, vibration and notification settings
Go to setting / sound and notification / General / phone ringtone. Select second sim card / select ring tone. Now you need to add ringtone with no sound. You can modify existing ring tone to no sound.
You need to root mobile. There is ringtone and ringtone_2.
You can find under media/ internal / audio / media / here are the list of ringtone. Change it to silent ring tone.
This is the only way. There is no setting to silent one sim card on android.