segment2 in DFF change to required base one the value of segment1 in the same DFF

BSA setup a DFF on Order Form.  The DFF is context field base on order type. ( see attachement  for detail setup ).

The DFF has 2 segment

1) eligable

2) NPI

I was ask to use form personalization to :

1) if user select Yes for eligable field, NPI field must be required.    When user click OK in DFF form, message should pop up and said "please enter NPI value if  eligable is set to Yes" and NPI is NULL

2) if user select No for eligable or leave that field empty, No check when user click OK in DFF form.

I am not sure this can be done using Form personalization.  I FEEL that this should be part of the DFF setup.... Maybe I even have to modify the CUSTOM.pll for this purpose...