First off:
The aria extreme is Cardbus (32 bit PCMCIA). There is no way to get this to work on your ibook (as far as I know). Yes, it may look like the internal airport card. No, it isn't the same connector in reality.

In fact, I am not aware of any means to get 2 radios into an ibook (running panther). As I recall, the D-Link USB adapter that's "Made for Mac" doesn't work in 10.3.

I have, in fact, used multiple radios on the same machine. MacOS X handles everything. When one interface goes down, or becomes unavailable, the next one receives the traffic. However, you must realize that since you will be on different networks, your IP address will obviously be different, and that because of this, the transition can't be "seamless," because all sessions to radio 1 will be lost, and radio 2 with a different IP will have to recreate them all.