Hi,
Currently one of the functions is private, so you'll need to modify the source slightly. The changes needed in tk2dAnimatedSprite.cs -
public float ClipTimeSeconds
{
get { return clipTime / currentClip.fps; }
}
Just add it somewhere in the file and you'll be good to go.
Once you do that, when switching animation, all you'll need to do is
animatedSprite.Play("animationSW", animatedSprite.ClipTimeSeconds);
This will start the other animation at the current offset into the animation. Assuming both of them have the same FPS it means it'll end up on the same frame.
Hope that helps.
That change above will be in the next 2D toolkit release.