Mini-display port MBP dual monitors
In theory Displayport 1.2 supports daisy-chaining and hubs, which would support this, provided you have a Mac that supports Displayport 1.2 (I think it's just 2011 Macs and later, but I couldn't find a definitive answer). However as far as I can tell, there aren't any monitors that support daisy-chaining available, nor are there any hubs expected until sometime in 2013.

At the moment you're restricted to the adapters that concatenate the two displays, rather than present them independently. An alternative might be a Displaylink adapter, which allows you to connect a monitor via USB. It does limit performance however, but how much this impacts you depends on what you're using it for.