Two things. One, it doesn't have to ride so low on the flap that it holds the flap against the part of the bag holding most of the contents. Both of mine are worn high in my dominant side (my right) with the horn riding on the upper part of the flap. The flap really won't open accidentally or randomly between the position of the horn and my arm, but, it will open when deliberately lifted and the horn rolls upward slightly.....
Which brings us to the second one, which,,,, I'm sorry,,,, seems painfully obvious,,,, they can still move independently of eachother. Need something in the shot pouch? Slide it forward of the horn. Need powder move it forward for access. Mine are carried fairly high, while shooting at the range they are about 3 o'clock on my body, but the horn still get swung forward for use, then put back. When moving through the woods I push them both back around 4 o'clock or so,,, and swing them forward for use. This keeps them from being caught up in "stuff" and from dangling forward if I bend forward.
Hope this helps