i think it just depends on the usb 3.0 controller which should be from nec..
maybe the chipset is also incompatible.... i would ask again what's the point of the problem.
the point is:
X58 48 lanes = 16+16+16, using two graphiccards there are still 16 lanes for USB3.0
Z68 16 lanes = 8 x 8, using two graphiccards you have a problem to transmit the USB3.0-data (internal using PCI-e-Bus)
so, if there is enough space for some extra data, i think there is no problem... but if you are really using the cards there might be a problem.
Therefore my question is anybody here using this card on a Socket-1155-system