How to show camera and LVGL8 in frame buffer at the same time ? And how to use two frame buffer mode in LVGL 8?

I use LVGL 8.0.2 on my embedded linux board,
I can get camera data to show on MIPI LCD via linux frame buffer,
and I can show LVGL on MIPI LCD too.
But I have no idea how to show both camera data and LVGL,
can anyone help me?
Furthermore,I want to use two frame buffer mode to show LVGL 8,
I had got some code from Internet which used on LVGL 7, but when I use it on LVGL 8, it not work well,
just like this.