Ahoy there! Here be me piratey advice. TL;DR: Aaarg, ye should mark all these 'ere words.
(This could be applied to any story, really. Although, it is oversimplified for brevity>) Simply put, if you had a situation where two or more of your characters could theoretically jump in and solve the problem, you may be experiencing character bloat. If your characters never disagree or offer new ways of handling a situation, you may be experiencing character bloat. This is not a one size-fits all sort of diagnosis, escpecially if you want to do this sort of thing on purpose (a lot of distopian/sci-fi have the bad guys virtually interchangeable, ala Stormtroopers in Star Wars), but if you're going for a cast of dynamic, thinking-for-themselves people, you'll want to avoid the taste of sameness.
As for Arrea and Kate, there are three ways you can atune their dialog and behavior to avoid any future wishywhashy, because you are pretty far in the story as it is and it would be a shame to go back and rewrite a lot of scenes:
1. (The Easy Route) Have one of them sacrifice one for the other in a pitched fight. You could even foreshadow it, adding spice to the blow. Keep in mind, this is pretty trite. Your choice over who lives and who dies should not come easily, but killing one as a plot device is in your writer's arsenal nevertheless.
2. (The Middle Ground) You could choose one and give her an accent. This will seperate her dialog from the other's, and you can pepper in some local slang/curses to help readers of that demographic identify with her. It also adds to her character. Where did she pick up the accent? Did she grow up there or did she watch too much Doctor Who growing up? This could also help you reach readers who like language play and language representation.
3. (The Difficult Road) This is one path I am loathe to recomend because it is no easy task: Rewriting. This is three fold. On one hand, you could simply write out one of them and consolidate them into one character. The easiest to write out would be Arrea, because Kate has history with other characters--and it would be a crying-shame to miss that powerful "You ruined my life!" moment. On the other hand, you could simply replace one of them with another character--perhaps giving even more purpose to Dulla-my-dude, or giving life to a new cannon-fodder-like character that readers aren't required to fall in love with. On the uh, third(?) hand, you could have them bonded together in the story. For example, you could have a curse or a spell go wrong and the two mythics are fused together! This is a long shot of course and can't just be inserted in, but it does offer the potential of being a nice plot hook and point as well. The only downside I see with a perma fussion is that it will take away from Hashi's, ahem, current two-for-one situation in the long run. However, if you spin it like a foible/mirror, you could get some nice literary contrast (eg. Hashi having growing pains with his other half, but Arrea/Kate flourishing together after they get over the fact that they share one body.)
As for my recommendation, I recommend you do what you think is best for your story! I don't know what you have planned for it, and you may decide something totally different that really surprises me and your readers. These are just my suggestions. Every story and every writer is different, so do what you're comfortable with :)
Most of all, have fun with it!! Chances are, if you don't enjoy a part of your story, neither will your readers.
Hope this helps a little... even if it is a bit of a book! (Palm/Face)