You could. You can also take 2 zyrtec a day - 12 hours apart. However, keep in mind that if you start an antihistamine after symptoms have presented, then it could take several days to catch up.
Mixing antihistamines can be VERY VERY dangerous. I would suggest you call your local Poison Control Center to check with them if you feel you MUST mix drugs. If you have a pharmacist you can talk to about this, they may be able to help you.
