Are petunias killing my pansies?

I planted window-boxes full of pansies and petunias, and have been deadheading them, watering them regularly and using MiracleGro. Thing is, we've noticed that the pansies are on the verge of dying and the petunias are HUGE and we're wondering if perhaps they are dominant and are in effect killing the pansies. I have the same kind of pansies in the backyard, and they are flourishing, but I find it really weird. Please help shed light on this for me - thanks!