Whoops, missed that bit - the French player should nominate which sides of the BUA he is holding, up to half of the figures within the BUA can man one side. This matters for firing only, it doesn't effect any charge/melee.
When the Austrian player makes his roll to charge home, the French player makes a "To Stand" roll for each unit within the BUA. If both stand then both will fight in the subsequent melee phase, their actual position within the BUA is of no matter - think about it as a game mechanic. The largest unit would get any out-numbering factor if applicable, and each would make their own melee roll. Only the highest is counted, same for the attacking Austrians.