It appears that your speakers still retain the internal passive crossover network when the switch is thrown. If this is indeed the case (and this is very important since running a tweeter without a high pass network will destroy them), you should be able to just hook up the amps, adjust the relative levels, and your'e good to go.

I would hook up the amps even if they don't have level controls to see which one of them is too loud. The louder one is the one you will have to use a volume control with. A simple potentiomenter that you can buy at RadioShack will suffice. A 10K ohm audio taper or thereabouts will work fine. If you don't feel comfortable with soldering and drilling, a passive preamp will also work - just keep the interconnects coming from it's output very short - under 6 feet.

I don't have any specific preamp to recommend since I generally build my own stuff, and only buy "store bought" when necessary, like a DVD player or something like that.