You could use the AfterUpdate event procedure of the text box to assign a
value to the combo.

However, I'm not sure it makes sense to store both fields in your table.
Would there ever be cases where the text box could be blank, but the combo
should show Yes? If not, omit the combo's field from your table. You can
still have a text box on your form, with this in its Control Source:
   =IIf([Text99] Is Null, "N/a", "Yes")
substituting your text box name for Text99.
This way, it can never be wrong.

If you want to go ahead and store the value using the AfterUpdate event of
the text box anyway, see:
   Calculated fields
at:
   http://allenbrowne.com/casu-14.html