Spreading out the contributions is a rule imposed by your employer and your employer's payroll department, not by the tax code; I can only speak to the tax code. Of course if your contributions are being made by deferral from your paycheck, you can't defer any more from a particular paycheck than would otherwise be included in your paycheck.
If you make any contributions directly to your HSA, not through your employer, you can divide your contributions up into as many deposits as you like. The benefit of making the HSA contributions through your employer is that the amount deferred from you paycheck to the HSA will not be subject to Social Security or Medicare taxes.
