Weightgainers tend to have a higher carb/protein ratio as they pack it a lot of carbs to help you gain weight. Also if you aren't careful many of the weightgainers have a significant amount of calories that come from sugar. Furthermore i think most weightgainers (including N-Large) are generally made up of a single type of protein (whey protein) while Meal Replacements tend to generally be made up of a protein blend (whey, casein, egg etc).
To answer your question, I don't think there is a need to be on both at the same time. Remember both of these products are substitutes for food (ie if you can't eat a whole lot or for convenience purposes). It also depends on your body type which one you use in my opinion. I tend to put on fat easily, so when I was using N-Large (which is one of the better weight gainers out there) I put on a lot of fat. Just my .02 hopes this helps.