Recommendations for an active, directional RFID sensor
What about using the difference between the signals from two antennas? 

In theory, if two antennas are exactly the same distance from the target, then the two signals should cancel each other out if subtracted.  If they are slightly off, then they would be slightly out of phase, and they wouldn't cancel.  

You could only use this technique to find the angle to the target.  You would have to rotate the antennas to actually read the RFID (because their would be no signal when you were pointing directly at it.  

(This technique is used in Ham Radio circles as a poor man's way to find people who interfere with radio repeaters so they can be reported to the FCC.)