We fabricate independently tunable tunneling barriers on a GaAs/AlGaAs heterostructure using a conducting tip of an atomic force microscope (AFM). In a humid ambient, a negatively biased AFM tip induces an oxide beneath it, which oxide depletes a shallow two-dimensional electron gas (2DEG). We exploit the depletion property of the oxide and use it to electrically isolate different regions of the 2DEG. In particular, we manipulate the tip movement to generate two oxide patterns whose combined geometry defines two quantum point contacts, in series, with a quantum dot between them. Our device not only permits independent tuning of the transmission probability of each quantum point contact, but it also allows us to observe Coulomb-blockade oscillations when both contacts are tuned into the tunneling regime.