These daggers have been bugged for like over a year, you couldnt even transmog the non legendary versions. Now that legendaries can actually be transmogged you cant even do it because of the same bug. Is there any word of this ever being fixed?
Hello. As mentioned in the Known Issues thread, this process ended up being a bit unintuitive for some players.
To choose the one you want, right-click the dagger appearance. This changes the appearances between your main hand and off-hand weapon.
This is similar functionality to how Druid Artifact appearances work.