is there a way to stack 3d objects like propeller hat and cancer perhaps

No the only items you can stack on one joint (head) is the hair.
But the 3D items like propeller hat and cancer stick no, one 3D object per joint.
but you can get the "Swagtastic" item which includes caner and a swag cap.

So for now you can't stack 3D items but maybe someday it will be possible.

You can wear 2 3d items at the same time as long as they're not on the same bodypart. So you can he 2 gloves,2 shoes, a hat and more, for example, as they are not on the same bodypart.