Are you sure you want to do this, because a user can select a combo value and that can enable the first textbox. Then user can delete the combo value and then what? So there may not be much point given that anyone can undo an entry anywhere in the "stream" of interaction thus basically nullifying the whole constraint. Trying to be so controlling is usually not a good thing. If you want to ensure data integrity, that can be done at any point before the record is saved. If that's not the purpose, then what is it?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.