Another method would be,
- Duplicate the animation twice, renaming them appropriately.
- Open the first duplicate and set the red time-position slider to the frame you want to split at
- Right-click the slider and choose “Remove frame 0 to frame X” (where X is your split-point)
- Open the second duplicate and do the same thing (you don’t need the slider but it helps find the right spot)
- Right-click again but this time choose “Remove frame X to frame Y” (where Y is the last frame)