Can I have two separate meshes in the same object? I have a head object that did not come with a set of teeth. I added a mouth object into my collection, but now I want the mouth mesh to be incorporated into the head object, while still retaining the mouth mesh. 

The reason for this is so that I can animate the mouth and the teeth individually, but have them operate under one shape key change. 

What I have:
|collection
- (yellow triangle) head
--- (green triangle) head
|
- (yt) Mouth
-- (gt) Mouth

What I want:
|collection
- (yt) head
-- (gt) head
-- (gt) Mouth


Unless there is another option? I am aware of drivers, but those won't work for the application I'm developing, since I need to use Three.js
Make the 2 meshes with their desire blend shape, then merge both meshes and now the 2 blendshapes should be in the shape key list, make both of them 1 of influence then create a new blend from mix.

 

Hope it can help you, im not 100 % sure its what your are looking for.