16x + 16x means that in Dual Crossfire both GPUs will run @ x16, so, yeah u can do it.
And take a look of the manual of the mobo in the page 37.
"...If you intend to install TWO graphics cards for CrossFireX mode, make
a. these graphics cards are of the same brand and specifications;
b. these graphics cards are installed on PCIE_E1 (1st slot) & PCI_E4 slots(3rd slot).....