It is perfectly fine to mix N2shampoo with other shampoos like Head and Shoulders but if you have N2shampoo, that's all you really need.
N2shampoo is a salon quality shampoo that has anti-hair loss ingredients added to it. It has all the same cleansing aspects of regular shampoo, just with added anti-hair loss ingredients. I use N2shampoo as my daily shampoo and use Billy Jealousy Conditioner as my daily conditioner.
It's best to use N2shampoo everyday so if you want to use head and shoulders as well, I would use 1/2 n2shampoo and 1/2 head and shoulders and shampoo your hair with that. There's no sense throwing out the head and shoulders you have on hand so use half of each until you run out of head and shoulders and then just use n2shampoo.