You can refer the first parameter in the second parameter using .$FLEX$.FIRST_PARAMETER_NAME.
Following on from the above reply, just to elaborate. you would create a flex value set based on a table and in there you would put the following in the where clause:
:$FLEX$.YOUR_FIRST_PARAMETER_NAME is null
then if you select a value in the first parameter, the above will cause the flex value set to return no rows and hence will become disabled.