You ahve teh main idea it just needs a couple of adjustments: instead of controling the sub level after the amp, place the sub vol after the main vol, and then run both vol control outputs to the amp. this way you are only modifying the low level input of the amp, instead of the high level side (which would require volume controls that can take the wattage--$$).
you will also need crossovers to block the signals that the speakers shouldn't/can't produce. there are 3 ways to go with this. you can either buy a active(powered) crossover to divide the signal before the amp, or you could buy what is known as a passive(non-powered) crossover to place inline between the amp and the speakers, or you could build your own passive crossovers.
If you choose to build your own there are a lot of resources for this, the first one that comes to mind is partsexpress.com--they have almost all the parts that you should need.
A quick find: how-to guide to building crossovers: http://www.parts-express.com/resourc...-crossover.cfm