@Yuriy It appears that the callflow ring_group has the option "fail_on_single_reject"; which looks like will do what you want.
https://github.com/2600hz/kazoo/blob/master/applications/callflow/doc/ring_group.md
@Yuriy, sorry I think I understand now... You want the mobile (second) device to drop/not ring if the the first device rejects the call?
I am not sure that's possible.
@Yuriy if I understand correctly i don’t think you want a ring group but chain 2 devices together on the callflow. So the first device rings and if the device rejects the call before answering callflow will move on to the second