They will tend to use different techniques in obtaining narcissistic supply. For example, they would tend to use their sexuality, their physique, seduction, etc. They will belittle each other and exaggerate their own achievements and qualities. Narcissists often modulate their sense of self-worth by berating others. Yes - IF they are each others' sources of supply.
Narcissists manipulate ONLY sources of supply.
They lose all interest, in frightening abruptness, once they have exhausted a source. Accommodate them, flatter the, adore them, admire them or get out of their way - and fast. They are vindictive. They are aggressive. They are emotionless.