To dissimulate is to pretend not to have what one has. To simulate is to feign to have what one doesn't have. One implies a presence, the other an absence. But it is more complicated than that because simulating is not pretending: "Whoever fakes an illness can simply stay in bed and make everyone believe he is ill. Whoever simulates an illness produces in himself some of the symptoms" (Littré). Therefore, pretending, or dissimulating, leaves the principle of reality intact: the difference is always clear, it is simply masked, whereas simulation threatens the difference between the "true" and the "false," the "real" and the "imaginary."

The Precession of Simulcra, The Divine Irreference Of Images - Simulacra and Simulation (1988)