Stacking Reta and Tirz??

Newbie here, and stacking Tirz with Reta was news to me. What's the upside of using both?

The reta plus tirz stack is the best combo I've found. Tirz alone beat reta alone in my case, but together the combo is doing great, 2 months in and it's been great. Only real downside: my dog allergy got worse, I can stay near them a shorter time before reacting and hives show up much faster. It's still totally worth it.