Generally no — don't routinely combine them: Zyrtec (cetirizine) and Allegra (fexofenadine) are both non-drowsy antihistamines doing the same job, and the NHS says not to take two antihistamines together unless a doctor tells you to.
Cetirizine (Zyrtec) and fexofenadine (Allegra) are both antihistamines — they work the same way, by blocking histamine, the substance in the body that triggers allergy symptoms (MedlinePlus). Both are also among the antihistamines that cause less sleepiness (MedlinePlus lists cetirizine and fexofenadine together in this group). Because they do the same thing, taking both at once usually adds little extra benefit while stacking the same side effects, so the NHS advises: "Do not take 2 antihistamines together unless your doctor advises you to." The NHS notes doctors sometimes do recommend two different antihistamines together for a few days — for example, for a severe, itchy skin rash — but that's a specific medical decision, not something to do on your own. If one non-drowsy antihistamine isn't controlling your symptoms, the NHS suggests it's worth trying a different single antihistamine rather than combining them.
Stick to one at a time at its labeled dose unless a clinician has specifically told you to combine them — a typical adult OTC dose is one cetirizine OR one fexofenadine once daily, and you should not double up to get a stronger effect. Watch for drowsiness: both are labeled "non-drowsy," but the NHS and MedlinePlus note it's still possible to feel sleepy on either, so don't drive or use machinery until you know how you react, and it's best to avoid alcohol (it can make you feel sleepy). Combining them mainly raises the chance of side effects like sleepiness, headache, or dry mouth without a clear added benefit. Talk to a pharmacist or doctor before taking both — and especially if you're pregnant or breastfeeding, are giving these to a child, have other health conditions, or if your allergy symptoms aren't controlled on one antihistamine alone.
This is general reference, not medical advice, and not a guarantee of safety. Interactions depend on your doses, health conditions, and other medicines. Always confirm with your pharmacist or doctor before combining products, and follow the dosing on each label.