Yes, you only need one but they say 2 will give you 1-2 frames per second more.(actually 2nd is for a third card)
Yes, after the OS sees the 2nd card enable it in CCC.
No, it has to be enabled in CCC.
Should be OK.
NOTE: Using two crossfire bridges for two cards can have unpredicitable results!
What mhelm1 said about using two crossfire bridges giving you 1-2 frams per second more is not correct. In some cases it has caused an increase in frame rates, in other cases it has caused a decrease. In my case it actually caused my system to be unstable and my games would crash now and then. I suggest you do a bit of testing to determine if one or two bridges is better.
As for CCC settings you need to select the enable option under crossfire.
I did the crossfire and my case temps went from 32 to 44. With the P6XD58 the two PCIe slots sit almost on top of one another so the cards basically can not breath.
I did a slight over clock and right of the bat on 3dMark I hit 86c and so shut the whole thing down. Prior to the OC, in just stock xfire mode, I saw about a 20% increase in 3dMark scores, not really worth the hassel and expense a this time.
Maybe later if I get a bigger case with better cooling. Or another option would be to try some of the other slots....i suppose i could put one in the 8x slot, though I am thinking that I would loose something there, after all it is only 8x instead of 16x, not that I am sure what that means, hahaha....
Anyway the temps put me way off and for the money, I dunno, like I said maybe later. A bigger case is a plan in a couple of years, the case I have now is to small to take a really monster HSF, so crossfire would be one more reason.
But it was really, really easy to configure. Put the card in, connected the ribbon and booted up the system. Catalyst loaded right away and asked if I want to set up crossfire and stepped me through the setting choices, where were few. I was hoping for more increase performance at a lower power cost. Oh well, it was fun and the card is returnable....
Top two card slots are X16 and the bottom is X8
With 2 cards in the top two slots both work at X16
With three cards in Top X16 second and bottom slots X8
Only so much bandwidth on the slots
He can still crossfire the 16x with the 8x without a real performance loss, right?
Still though, crossfire bridge probably wouldn't reach. Haven't seen too many 16x16x8 mobo's, probably because there aren't many on AMD's side at newegg.