Well I'm not sure about most brass instruments - trumpet, flugel horn etc - perhaps they can be played by either hand, but are just usually played a certain way. A trombone however, has the slide on a certain side (not sure which!) so it wouldn't be ambidextrous as far as I am aware.

Flutes, recorders and clarinets etc have the left hand as the top hand and the right as the bottom, but you need to use both to make the range of notes for a scale. Hence it isn't one playing the melody and one the harmony - you can only play one note at a time!

Allie