I was doing the "It Takes Two" the other day, and apparently, I was digging so quickly that the other person was having serious trouble getting the "Help Dig" option in time. We managed to make that work exactly once. This seems to be because I wound up getting enough of the digging/gardening skills that it made the process too fast by the time I actually made time for the quest--I was able to help *her* dig easily enough. I'm not sure if there's an easy solution--maybe a plus-two-second window for "helpers?"