Just to chime in here, @mercrutio is absolutely correct… I just wanted to point out that most 13.56mhz chips are designed with anti-collision as a feature. This means there will not be any “interference”… both chips can be in the field and be read just fine because they are designed to be. What you might get though is unexpected results from readers and software not designed to properly accommodate multiple chips in the field. Most commonly this results in one being read and the other or others not being read.
The best way to control this is to ensure you control your presentation of the chip to reader. Most readers will not have a problem picking up one only one chip if the xNT is between the thumb and index metacarpal, and the other device is several centimeters away.