Is it possible to retrieve the selected value of an automation workflow parameter that has the acceptable values generated via a script?

Is it possible to retrieve the selected value of an automation workflow parameter that has the acceptable values generated via a script? 

I have 2 workflow parameters.  Both acceptable values generated via a powershell script.

Param1 and Param2.

I would like to use the selected value of Param1 to calculate the acceptable values of Param2.  I cannot seam to retrieve the value that was selected from the list (Param1) until after they click ok and run with workflow.

This is kinda like a bound list but not really but I would take it if someone knows how to do a bound list of workflow parameters.

Maurice