it's Wrong,they just the same camera....
//on start_time
public Texture mtexture=new Texture(256,256);
public framebuffer fb=framebuffer(w,h);
public World world=new World();
//on frame_time
//first rendering
cam.auto(world, cam, Objec3D1);
fb.setRenderTarget(mtexture);
fb.clear();
world.renderScene(fb);
world.draw(fb);
fb.display();
fb.removeRenderTarget();
//second rendering
cam.auto(world, cam, Object3D2);
fb.clear(back);
world.renderScene(fb);
world.draw(fb);
fb.blit(mtexture, 0, 0, 700, 8,256, -256,device_width/2,device_height/4,100,false);
//-256 must be negative,otherwise the second camera would be revise
fb.display();