can anyone help....

I want to use data validation to allow a drop down list in a cell but only
if certain criteria are met.

e.g. If A1="yes" then show yes in the validation cell, if b1="no" then show
no, if not then shown the dropdown list for yes/no

At the moment i have the validation in cell d1 as a list with this formula:
=IF(OR(A1="",B1=""),YESNO,IF(A1="YES","YES",IF(B1= "NO","NO",YESNO))))

With the name of my list being yesno

thanks for any help