Another newbie question, please. If Field x has data entered into it by the operator, y needs to have data entered also and vice versa. How can I put a warning dialog in so if the operator enters data in x but not y they get a warning before they can proceed?

Using OnRecordCommit script trigger, check which field has value and show warning accordingly.

I'm a newbie and haven't dealt with Script Triggers. But, wouldn't OnObjectExit be more appropriate?

No, because as soon as you leave X you'll be prompted with the warning before you've had a chance to enter a value in Y.

You're absolutely correct as I discovered trying it. I will have to write a script to trigger a dialog if one or the other is empty but not if both are completed, right?

If you need to ensure data integrity, use field validation first and foremost.

Then you may add a script trigger to protect the user from Filemaker's annoying validation error message.