Besides the artifacts of course.

I'm grinding my way the level 90 dungeons, but find that characters without double action aren't very effective. It would be nice if I could get them an accessory with double action until I get their artifacts.

The only one I know beside Artifacts that give Double Action is Crayanne's Time Bracelet accessory, but that's character-specific too...

Nope (or at least I'm not aware of their existence). But if you want my opinion 90% of the characters in your party wouldn't be effective even with a triple or quadruple action.
And IMHO all the useful characters for the 90 lvl dungeons have got the skill (Mylen, Will, Yuela).

There's an accessory that gives Swift. Which isn't the same thing, but the best I know.