mix / overlay different 3D elements in a jit.gl.render

sorry but i searched a lot here before posting this.

(total 3d beginner)

i have different elements
- a "sky", which is a plane in the "background",
- a 3D model,
- a "sea" which i make with the pool 3d nurbs example
(and the next "layers will be : rain / snow, with particles)

For the moment, i have 2 different jit.gl.render. i would like to "mix" them in a single one, and also place them on different layers.

but ... how ?
if i send a "drawto smthg" from the 3d nurbs patch to my 3d model patch, it just blinks and doesn't work at all.

i'm stuck...