with one of them hidden so nobody can see it,
Remember you will have users without CSS. Don't let them get caught in the trap.
These will include those using text browsers and those using other technology which sits atop browsers like screen readers and refreshable braille devices.
You can avoid hitting those people by making sure your "hidden" fields have proper labels warning humans not to fill them in or something.
Now it was in Dutch and said "Nee invullen" and the first several times I tried to post I misread it as "Niet invullen" (don't fill in) so it still got me a few times. Never underestimate the stupidity of a user filling out a web form, even when (or especially when) they are rocket surgeons. Everyone's IQ seems to automatically drop at least 10 points once they start filling out a web form, and they also become semi-illiterate too.