Carl,
What I want is this.
1) The user presses a button, (say "Animate one"), that animation plays until the pause point (1 moves to the right).
2) The user presses another button, (say "Animate two"). This causes two things to happen:
a. The previous animation plays the second half (1 moves back to the left).
b. Once this has completed, the selected animation plays (2 moves to the right).
(I made this codepen to try to solve the mechanics of a more complex animation I am creating, where each button gives an animation about a different subject, and the second half of each timeline is just making a neat transition between the different animations.)