Ok not necessary? Both Cetirizine (zyrtec) and Diphenhydramine (benadryl) are antihistamines. While there is little harm in taking the 2 together (except drowsiness in some people even at usual doses), it should not be necessary. If symptoms are that severe, see an allergist for a better treatment plan.
Maybe. Depending on what you are treating, i would space them out, take zyrtec in the morning and Benadryl (diphenhydramine) at night. Both are sedating though, so be careful driving or operating machines. Not sure what you are treating though. I can see that with urticaria but for nasal allergies, i would use a nasal spray instead of a second antihistamine.