You can add SLA warning in same workflow as SLA breach if you haven't do so. What I use is that I created Default SLA workflow, I use SLA percentage timer. Here is example

The percentage timer will verify time requirement from SLA Definition. I set 50% and 75% that will trigger SLA warning. Then last 25%(100%) will trigger SLA breached. In Create Event is where it trigger event sla.warning. sla.warning will trigger SLA notification. In SLA Warning notification, make sure under "When to send" tab have Event name is fired and event name like sla.warning that is triggered from SLA workflow and condition that "task.assigned to" is not empty.