Hammers - the bank figure is exactly the same whether you record it as multiple bets or a single bet. eg. $10 on allup paying $2x$2xlose - initial bet is $10 so bank is -10, pays $20 so bank now +10, next bet $20 so bank back to -10, pays $40 so bank now +30, next bet $40 so bank back to -10, loses so bank stays at that.

Reenster - an allup is not a single bet - it is multiple bets that the TAB places for you. Each bet pays a dividend (if it wins) that is reinvested into the next race. This is the way the TAB accounts for it so why should we look at it any differently?

Of course this raises the question of would a serious punter place all-up bets automatically and lose control over the reinvestment? I certainly wouldn't because you have no way of knowing when you place the bet what the divs for 2nd and 3rd legs are going to be. Say you have an allup with $10 on first race which pays $3.00, 2nd race pays $4.00 but your last selection gets heavily backed and at jump time is only paying $1.10 the place - do you really want to risk your $120 just to get an extra $12 if you don't feel the horse deserves to be that short. Alternatively maybe the jockey for 3rd leg gets injured in 2nd race and is replaced by apprentice who has never ridden the horse before - again do you want to risk your $120? For me it makes more sense to place the bets individually so you can take your profit if something happens to change your mind about the 2nd or 3rd runners.