Basically a backsword can only be worn in the first hand.
Taoists (reborn and not reborn) are programmed to have only one hand.
TQ designed it this way because it would be an advantage for a tao to have a shield. Basically because, for a trojan to use a shield he'd have to take off his second weapon making him pretty weak. If an archer wears a shield, he loses his ability to fly. If a tao wore a shield, he wouldn't be losing anything, only gaining stuff.
What TQ needs to do is program a backsword to be considered a two-hander, so that taos could have a 2nd place to equip a shield if they wear a blade or a club or what not.