I don't know a lot about UHF or Wide band RFID since we only sell 125 kHz tags and readers, but how do you plan on detecting the approximate location of the tag? It is my understanding that a tag is either in reading range, or outside of reading range, and this is the only information you can get in regards to its position. Even with two antennas, the only information you get is whether the tag is within range of one reader, the other reader, both readers, or neither. You may also have problems with the readers interfering with one another unless you poll them so that only one is active at a time.
Are you sure RFID is a better solution than GPS in this application?