Using macros in field definitions to assign alternative forms

Hi,

I’m trying to add 2 fields using dropdown list control to a document type so that under the form tab, the user can assign a single bizform to the page, as well as assigning an optional alternative form.

I’ve managed to get the first field in by putting in the below to select a form name:

;Please choose a form
{%SiteObjects.Forms.DisplayNames#%}

What I’m having trouble with is creating the alternative form selector. I can use macros to get all alternative forms, but I need a way to filter that list so that it only applied to the chosen form in the previous field. I’ve already set the alternative form field to enable only if there is a value in the form field. Can this be achieved?