Why can’t both your grandparents and he celebrate the same achievement on different occasions? Alone time and time with different friend and family groups outside of a romantic relationship is definitely healthy. You two should explore why this bothers him and establish some boundaries.

While they’re not requirements, most dogs require some level of training and grooming whether you enlist the help of a professional or do it yourself and it is 2 times the work with 2 animals.

I think it’s a common assumption that two dogs will “tire each other out” and/or entertain each other, but this depends greatly on their personalities and age gaps and it’s not a cop out from actually playing with your dogs, walking them, and providing mental stimulation. Outside of special cases such as littermate syndrome dogs typically find human interaction more rewarding which is a trait we selectively bred into them over generations.

They do if you consider taking them to training classes, groomers, and the vet separately. Some dogs are difficult to walk together for the first year of being together and require separate walks as well. So I would say two dogs definitively take more time than one, especially in the formative years, and then in the latter years as well.