Can I take diphenhydramine and loratadine together?

Yes. Taking Loratadine in the morning and Diphenhydramine at night will help you avoid the drowsiness side effects of Diphenhydramine and help you if the Loratadine is not controlling your symptoms for the full 24 hours.
Benadryl/Claritin . If you have really bad allergies yes you can take them together. But you should see an allergy doc if you need to do this frequently.