Thank you for your question. There have been reports of patients taking 15-18 times the recommended dose of Zyrtec (recommended dose is 10 mg) and having some adverse effects (restlessness, irritability, somnolence). Taking two 10mg tablets should not cause you to have any serious problems. HOwever, if you notice anything unusual or have any negative side effects, you should contact your doctor or visit an ER to rule out any possible complications.
Best of luck,