I tried it, no success again.
My method
1. Turn off Atari.
2. Side2 switcher to upper position.
3. Turn on Atari.
Atari boots to SDX.
4. Press Help+Reset.
5. Press L key,
6. Select atr image in Side loader and press Space to mark file as D1.
And now next problem, how to get back to U1mb bios ?
When I press Help key this action deselect the selected atr file.