Canvas 3D drawing using both 2D and 3D context

Create the text as a texture using canvas 2D, then render it in 3D. See here for a tutorial.

As stated, you cannot do this.

However you can put one canvas on top of another and draw to them separately. I've done this before and it can work out quite well.

No, unfortunately not.

The HTML 5 spec says that if you call getContext on a canvas element that is already in a different context mode and the two contexts are not compatible then return null.

Unfortunately "webgl" and "2d" canvases are not compatible and thus you will get null:

var canvas = document.getElementById('my-canvas');
var webgl = canvas.getContext("webgl"); // Get a 3D webgl context, returns a context
var twod = canvas.getContext("2d"); // Get a 2D context, returns null