How to make one OR the other parameter required? (but not both)

Hi all,

I've got a report showing expenditures by budget code. There's a multi value parameter allowing the user to choose any codes they want, but this can be tedious, since there's a couple hundred of them.

I would like to offer as an alternative a couple of "pre-packaged budget code groups" on another parameter. How can I set it up so that the user must choose EITHER something out of the full multi value set, OR one of the 2 or 3 pre-packaged options?

thanks for any tips,

sff