You cannot enable the ports on the crossfired card. You also cannot use card # 2 as the main card as I have found out after attempting to do this. I have a 4 monitor set up. 1920x1080 BENQ 22" x3 and one ACER 20" 1440x900 for hardware monitoring. I have two of these.
There are some bad reviews but I have only had to RMA one of my adapters out of stupidity. They work great. It is possible to plug the mini display port adapter in upside down without a lot of force so be careful since doing this will kill the adapter. I supsect that may be part of the reason why there are so many bad reviews talking about DOA adapters. I have used my setup with a 6990 crossfired with a 6970 and my current 7970 x2. I have had to set up my monitors in CCC at least 6 times for various reasons and everytime I still scratch my head when trying to figure out how CCC works. I find CCC can be a bit confusing when trying to set up multiple monitors so you should follow the guide the first time. Also, that guide is not entirely accurate. You CAN use different resolution monitors in an eyefinity display group with the 7xxx series gpu's
If your motherboard's bios has an option to choose your "init PCIe" slot, you can change which card is the primary card, allowing you to plug the monitors into the 2nd card (it'll technically be the #1 card with the change).
But as mentioned above, all monitors have to be plugged into the primary graphics card.
Interesting point about selecting initial pci-e slot. I do recall having an old gigabyte motherboard that had that option but I have not seen it in the last 3 motherboards that I have owned. Ignore that part of my original post.