you can do that if you tolerate both medications well. try to do without medication as much as possible especially toward the end of the cold because sometimes the backlash reaction from coming off antihistamines (like zyrtec) can make it seem like your cold is lasting longer than it should because it can make your nose stuffy when you are coming off it.
if you have a cold can you take zyrtec in the am and nyquil at night.