We noticed that at some (Skill) Events, the timer is located in such a way that it's not visible to the driver. So we got the idea for the second driver to have simple electronic countdown timer attached to the wrist. Similar to like people attach phones when jogging. 2nd driver starts it at the start, and than both drivers can see it and know the time left.
At one of the events we were told it's not allowed. But I can not find anything about that in the rules. Seems like if it's not allowed, than any hand watches should not be allowed, which is silly.