The bidirectional tag can refer to both bidirectional text (such as Arabic and Hebrew), and to bidirectional relations in databases and data structures, which are supposed to be covered by the bidirectional-relation tag.
I'm not sure what to do with them. Both topics are reasonably common. I would probably make the existing bidi tag a synonym of rtl. What to do with bidirectional? It should probably a synonym, but I'm not sure for what.