Disciple of Varashta - Separate support gems per Djinn command

Im currently planning my Varashta build for the league and saw you share support gems for different commands of the same djinn? Is this true? If yes, what's the point?

I was thinking of using one of the sand djinns commands to shred armour and the other to scale damage, but if they share supports I can throw my plans out the window. Why is this so limited?