There is an easy way to realize the two-way stretch which is adding two stretch actions. Though it can realize two-way stretch, the action of stretch is independent. The parameter needs some additional setup to realize the two-way symmetrical stretch.

1. Add Stretch Actions:

Add two stretch actions for parameter, and select the two grips of the two parameters as the relevant point of each action.

2. Modify Base Location: 

In the misc, modify the base location from “Start Point” to “Mid Point”, like the shown picture below. Save and quit the block editor.

3. Test Dynamic Block: 

For the convenience of watching the effect, we draw a vertical center line at the middle of the keyway, and then stretch the grip to the left side of the keyway or the right side. Like the shown picture below. As the grips moving, two-way symmetrical stretch realized.