Can it hurt to take two doses of zyrtec (cetirizine)?

Not likely: Zyrtec (cetirizine) is a new generation with a good safety profile. While the usual dose of zyrtec is 10 mg once a day for allergies, there is scientific evidence that higher doses even up to 4x the usual dose is helpful for severe hives. So, two doses of zyrtec is unlikely to cause significant side effects except perhaps a little drowsy.