Krita has a framework to allow painting assistant, it is the virtual version of physical tools such as rulers, compass... It allows to restrict the drawing to follow a given shape. It differs from the line/ellipse tools in that those tools gives perfect and regular results, while the assistant allows the artist to have some control on the drawing. The following blog post contains a video that show it in work:
The tool to manipulate painting assistant is limited, it allows creation and edition. This task would allow the user to merge two handles together, the idea would be to use it to create a polyline, or with two perspective assistant there would be the two faces of a building. So when moving a handle, if the handle move close to an other one, and the user release the mouse button, the two handles should be merged, unless the user is pressing the shift key, in which case no merging happen. A shift click on the handle should split the handle, ie make one handle per assistant.