Lateral patterning of a buried GaAs epilayer can be achieved during molecular beam epitaxy growth by in situ implantation with a high energy focused ion beam. Applying this technique, 30 keV Ga ions have been used to form small, highly resistive, regions in the backgate layer of a double two-dimensional electron gas (2DEG) structure. Independent contacts to the two 2DEGs were then achieved by selectively depleting out regions of the upper and lower 2DEGs with potentials on patterned front and backgates. In the resulting devices, the magnetoresistance of the two 2DEG layers was measured both together and separately. It was demonstrated that a bias of up to +/-50 mV could be applied, across the 20 nm AlGaAs barrier separating the 2DEGs, with leakage currents of <0.01 nA at 4.2 K. Finally, resonant tunneling between the two 2DEGs was observed when the barrier thickness was reduced to 7 nm. These results have demonstrated the success of this novel in situ fabrication route.