I am not an electrician. However, the first thing I'd do would be to spend some time figuring out how that circuit works. To that end, if you don't have one already, buy a voltmeter from a place like Radio Shack. You can use it to test for the presence of voltage before and after you flip your switch, etc. It's a handy safety device too. I always use one to make sure the wires I'm working with aren't live.

Getting to your outlet, it's reasonably likely that power comes into one of them. Let's call that one outlet #1. Power to outlet #2 probably piggy-backs off of #1. If #1 is switched on, then so is #2, and vice-versa. My guess is that you can make outlet #2 either completely unswitched or completely switched, but that you won't be able to split it without running new wire.

If power comes in to outlet #1, and you have a wire going off to the switch, then you should be able to rewire outlet #1 so that it's always on. Or, you could split it so that it's half switched and half unswitched.

Having said all this, the outlets may not be wire in the way I'm envisioning. Take some time to figure out the purpose of the cables coming into and going out of each of your outlet boxes, and then post back here with the details.

Jonathan Gennick
http://gennick.com