The two exchanges are separate: like Queen Victoria Market and South Melbourne Market. Your broker chooses which one to send your order to. Most brokers will have some kind of "best execution" process where, if there is a difference, they will choose the exchange where you will get the best price. If the prices are equal on both then they might choose the one with the greatest liquidity. With that kind of approach, most orders are going to end up on the ASX.
Once your order has been placed on one exchange then any orders placed on the other exchange won't reach yours. It can be very annoying if you are trying to trade a stock that doesn't have much liquidity, when you see trades flowing through on the other exchange and yours is just sitting there.
I don't know of any online brokers that let you specify which market to put your order on. Perhaps if you place your order by phone you could ask. You'll pay higher brokerage for a phone order because somebody has to put their hand on their mouse to make it happen...
AL3 Price at posting:
15.0¢ Sentiment: Hold Disclosure: Held
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