What is an advantage of vertically stacking horizontally polarized Yagi antennas?
Why Stacking antennas in a given plane sharpens the pattern in that plane. Two Yagis spaced vertically form a broadside array in the vertical dimension, so their combined pattern is compressed in elevation, adding roughly 3 dB of gain and pushing more energy toward the low takeoff angles that favor DX. The individual elements stay horizontal, so the polarization does not change.
Watch out Narrowing the main lobe in azimuth is what you get from stacking side by side horizontally, not one above the other. Selecting or combining polarizations requires separate vertically and horizontally oriented driven elements, which stacking identical horizontal Yagis does not provide.
Stack the direction you want to squeeze: vertical stacking squeezes elevation, side-by-side squeezes azimuth.