Currently, you can only apply one constraint to one object.
This feature is currently under development. It can be used in production, but keep in mind that it has limitations
A constraint is an entity that binds several objects in the scene, allowing one of them to control the positions of the other. Such a feature can be especially useful when you are creating animations that involve additional objects such as tools or weapons.
Constraints are created via the Commands → Rig additional menu.
There are two options: Constrain points and Constrain transforms. Which one to use depends on the kind of constraint you’d like to create.
This option applies constraint to the selected Point Controllers.
1. Select several Point Controllers
These Point Controllers will be controlled by the constraint.
2. Add a controlling object to the selection.
This object should have a Transform behavior i.e. spatial coordinates, rotations and scale. For example, you can use a triangle, a joint or a separate mesh object.
3. Select Constrain points from the menu.
The points will be constrained to this object, and when the constraint is active.
This option only works to constrain Point Controllers. For other cases, you can use the next option.
This option works with other objects that have transforms: joints, meshes etc.
1. Select two objects that have Transform behaviors.
Currently, this option doesn't work with any other number of objects.
2. Select Constrain transform from the menu.
3. The Transform dialog should appear:
4. In this dialog, set which one of the objects should be the parent.
After this, the other object will be constrained to the object you’ve chosen.
Using Constrain transform, you cannot constrain objects that are connected to Point Controllers. For them, only Constrain points should be used.
Once a constraint is created, it is automatically selected while other objects are removed from the selection.
It can be convenient to create a new Animation Track to place every selected constraints on:
1. On the Timeline, click the Add track button and set a name for the new track.
The selected constraint will be moved to this new track.
Once the constraint has been placed on a dedicated track, you can control it by switching fulcrum intervals (Alt + R) on this track.
If fulcrum interval is active, so are the constraints
You can also turn on constraints as you edit the pose.
1. Select the constraint by double-clicking its Track.
Alternatively, it can be done in the Outliner.
2. Go to the Object Properties panel.
3. On the Dynamic tab, turn on Active on frame.
4. Set fulcrum (Shift + R) on the corresponding keyframe.
If there is no key on the current frame, create one. Otherwise, you won't be able to control the constraint.
Also, constraint can be completely deactivated in the property editor.
By default, constraints are parented to the objects they control.
The video below shows how constraints can be used to improve animation: