True, but it makes the bike feel strange when your relative phase keeps changing. And makes it hard to keep both inside pedals up when going around a sharp turn since the captain won't know the position of the stoker's pedals.

Another approach is to change the crank arm lengths. Give the spinner long crank arms and it'll naturally slow down their preferred cadence - conversely, give the masher short crank arms and they'll tend to raise their cadence.