Normally, to see the sun and moon at the same time would still be possible on a ball-Earth. The sun and moon are not always directly opposite each other, with respect to the Earth.
However, during a lunar eclipse, it should not be possible. A lunar eclipse is explained in ball-Earth theory by the sun, Earth and moon being at 180 degrees to each other (i.e. in a straight line). If this were true, it should be totally impossible for anyone ever to see the sun and moon during a lunar eclipse. It is a rare sight, but it does happen enough for the phenomenum to get its own name - a selenelion.
The selenelion, in my view, invalidates the ball-Earth theory of eclipses, and puts ball-Earth theory in the same category as flat-Earth theory when it comes to explaining eclipses (i.e. there isn't a satisfactory one). Ball-Earthers go on to claim the reason that both sun and moon can be seen at the same time is atmospheric diffraction, but there's no proof of this claim, it seems to be just a catch-all excuse for every observation that directly contradicts (i.e. falsifies) ball-Earth theory.