Actually, they have this. You may have heard alot of WiFi buzzwords being tossed around as of late, like 802.11n or MiMo, which stands for Multiple Input, Multiple Output. Basically what this means is there are 2 recievers and 2 transmitters in each pci card/pc card/usb adapter/access point. This not only makes it much faster, but much more reliable. Problem is, the WiFi board hasnt set a standard for MiMo yet, so anything you buy isnt going to be WiFi certified, AND it will almost certainly not be compatible with other brands, or any 802.11n stuff when it DOES come out. Hope this helps
Note, you cant just stick 2 PCI cards in ur PC. its like pligging 2 network cables in, u have to choose one and stick with it, theres no way that i know of that can have it automatically switch back and forth depending on signal strenght, which would be relitively the same, since the antennae would be right next to each other anyways.