Messing with eyes is incredibly hard, and incredibly easy to make look terrible. The reason you can't find a tute on my site is that I don't recommend it.
However, if you're brave enough, I think it would go like this ...
Copy both irises onto separate layers, then combine them to make one complete round iris.
Copy the left (white) part of the right eye, flip it, and put it in the right part of the left eye; then vice versa, to essentially remove most of the existing irises.
Copy the new whole iris you made, so you have two of them, position them as best you can, then mask them in.