Can i take allegra and benadryl (diphenhydramine) together?

Yes, but why: Allegra (fexofenadine) and Benadryl (diphenhydramine) are both antihistamines. Allegra doesn't cross into the brain, so no sedation. Benadryl commonly causes sedation. Typically there is no advantage to taking the 2 together, but also there is no harm at usual doses.

Maybe: Depending on what you are treating, i would space them out, take Allegra in the morning and Benadryl (diphenhydramine) at night. 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.

2 antihistamines: At usual doses, there should not be significant side effects from taking these 2 antihistamines together. If a person is needing this much for relief, consider seeing an allergist for an evaluation.