User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-174.52.115.52-20150608063306/@comment-1359322-20150608065709

The Ex-Durandal is a fusion of both the Durandal and true Excalibur (7 Excalibur swords). And since Xenovia can materialize the Excalibur as a sword, dual wielding with both sword is kinda normal.