Determine which RFID is closer
It's a near-field RFID reader - it'll probably work up to 0.1 metres with a passive tag. Trying to distinguish between two tags in a 0.1m maximum range will be nearly impossible with the chips that are used. Also, with two tags "stealing" magnetic energy from the field, the range might be much much smaller than 0.1 metres. Data collision detection and correction is also a really big-deal and I doubt there would be any that allows two tags to simultaneously communicate with the reader.