Can i take one antihistaminic at moorning and another one at nigt? for alergic reactions.

Yes but...: Antihistamines can be taking twice a day safely if required but there are other treatments available. If by allergic reaction you are referring to seasonal allergies, the gold-standard therapy is daily nasal steroid sprays and/or antihistamine nasal sprays. Nasal saline rinses, avoidance measures, and if refractory to treatment, allergy shots can also be helpful. See an Allergist for more details.