Actually, the FAQ contradicts itself.  Based on the answer to this question...

Q. Are there any other fees charged when trading in multi-currency?

A. No. However, please be advised that there is a 2.5% premium when a trade involves orders in different currencies.  However, this is not a separate charge. It is already reflected in the price you see.


Based on that it looks like there are indeed separate order books, but that all trades are reflected on other currency book (minus a 2.5% currency commission - which quite frankly kills the competitiveness of the order).  So the moral of the story is that it's best to trade on the busiest currency.  If you trade in Thai Bhat, don't expect to get your orders filled.