Yes, you cant use more then one VA11A at the same time, but you can add a 2nd reciever to the single VA11A.
Did you have AHP before purchasing iWitness addon? Did you make sure you entered the new Product registration code for the iWitness software module? Goto Help / Product Registration and see if it says its registered.
Seems to work ok (even with ATI compatibility problem) without the Xray vision software installed, maybe try taking that off your PC and see if that helps.