Yes. However, if one is clocked slightly higher, it will 'downgrade' to the slower card's memory and clock speeds.
Just make sure your motherboard has two PCIe x16 slots that operate at either x16/x16 or x8/x8, and your power supply has the juice to operate a pair of cards in CF. One thing I'd mention in case you haven't heard it yet is to totally uninstall your ATI drivers, and clean out the remnant files using 'DriverSweeper' before plugging in your new card, and reinstalling the ATI software. Do these things and you'll be golden, and loving your setup.