Multiple overlapping animations on the same element in svg.js

I want to start a animation on an element while a previous animation is still active. However, calling animate() on the element queues the new animation at the end of the current animation.

For example, consider an animation where an element is being moved to a new position. Now, I also want to make it fade out when it reaches a certain position. The following queues the “once” animation at the end of the move, rather than at 80%.

rect.animate(1000).move(100, 100)
    .once(0.8, function(pos, eased) {
                 rect.animate(200).opacity(0);
               });

How do I make the element start fading out when it reaches 80% of the move? The API seems to be designed for chaining animations rather simultaneous overlapping animations.