WEP/WPA on the fly switching strangeness

I'm using WPA encryption on my MacBook currently, but I have a Nintendo DS which doesn't support this format. So, when I want to take my DS online, I have to switch my router to use WEP encryption for that duration.

As I want to continue being able to browse the web on my MacBook while I'm using WEP encryption, I've set up two wireless profiles on the computer, both of which have the same SSID [Network name], but use different encryption protocols.

What I'd like is for the computer to automatically detect when I change between WEP and WPA, and use the correct profile accordingly.

Here's where the problems start.

Switching from WPA to WEP appears to be fine - I just change the settings on the router, switch Airport off and back on, and it connects (although I'd prefer if there was some way of doing it without having to switch AIrport off temporarily).

When I switch back to WPA encryption, the computer can't connect. I check my network settings, and it seems the MacBook has overwritten my WPA password with the WEP key even though they are in seperate entries in the network access list. I then have to retype my WPA password in to get it to work.

Is there some way to stop the computer from doing this?