Limit Multi select of categories

[jasharen]

Brian,
I'd like to allow but limit multi select of categories. What I mean is, limit multi select to the same sub category that was originally picked.

So if I had parent category 1 and 2, and each had sub category 1-A 1-B and 2-C and 2-D, if I picked 2-D as my main category, I would like to limit multi select to only allowing 2-C to be picked.

I am ok with doing a file edit if this is needed.

Thanks!