I want to override the ModelAdmin. But I want to include the functionalities of two models into one. However I'm not considering merging the two models into one. Is there any other way in which I can override it and include the functionalities of both of my models into one ModelAdmin?
It depends on a relation between your models. Take a look to InlineModelAdmin:
https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.InlineModelAdmin