thanks to ego and thomas,the problem solved here is the code.
cam1.cam01=world.getcamera;//
//first rendering
world.setCameraTo(cam2.cam01);
cam2.move(world, missile01,2);
fb.setRenderTarget(mtexture);
fb.clear(back);
world.renderScene(fb);
world.draw(fb);
fb.display();
fb.removeRenderTarget();
//second rendering
world.setCameraTo(cam1.cam01); //without this,two views would be share the same camera.
cam1.move(world,tes1,1);
fb.clear(back);
world.renderScene(fb);
world.draw(fb);
fb.blit(mtexture, 0, 0,device_width*5/6, 8,256, -256,device_width/6,device_height/5,100,false);//-256 must negative,otherwise the second camera would be reverse
fb.display();
}