Bind a DropList required validator to a prior textbox entry

I am trying to gather the input for a day's schedule (start time, stop time, and a dept code). The droplist for the dept code has a blank as the first item. How would I bind a "required" validator for the droplist to the start time textbox, and then enable/disable it based on the existence of text in the start time texbox? These controls are in a FormView, and I have seven FormViews ("fvSun" - "fvSat").  There is also a Master Page being used. I have tried to use the TextChanged event, but I cannot seem to navigate to the specific controls within the FormView. Any ideas?
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